Add docstring for DefiniteIntegral

[codecov[bot]]

Codecov Report

Patch coverage: 88.54% and project coverage change: -0.98 :warning:

Comparison is base (2f401fa) 71.22% compared to head (94f0c0c) 70.25%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #475 +/- ## ========================================== - Coverage 71.22% 70.25% -0.98% ========================================== Files 80 81 +1 Lines 8331 8456 +125 ========================================== + Hits 5934 5941 +7 - Misses 2397 2515 +118 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/ApproxFunBase.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL0FwcHJveEZ1bkJhc2Uuamw=) | `84.00% <ø> (+0.66%)` | :arrow_up: | | [src/LinearAlgebra/blas.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL0xpbmVhckFsZ2VicmEvYmxhcy5qbA==) | `77.77% <ø> (ø)` | | | [src/Spaces/Spaces.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9TcGFjZXMuamw=) | `63.52% <ø> (-3.53%)` | :arrow_down: | | [src/Spaces/SumSpace.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9TdW1TcGFjZS5qbA==) | `70.44% <33.33%> (+0.77%)` | :arrow_up: | | [src/show.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3Nob3cuamw=) | `88.59% <78.04%> (+5.76%)` | :arrow_up: | | [src/Operators/banded/CalculusOperator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9iYW5kZWQvQ2FsY3VsdXNPcGVyYXRvci5qbA==) | `75.40% <80.64%> (+0.84%)` | :arrow_up: | | [src/Operators/functionals/CalculusFunctional.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9mdW5jdGlvbmFscy9DYWxjdWx1c0Z1bmN0aW9uYWwuamw=) | `79.48% <86.66%> (+7.07%)` | :arrow_up: | | [src/eigen.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2VpZ2VuLmps) | `96.42% <96.42%> (ø)` | | | [src/Operators/Operator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9PcGVyYXRvci5qbA==) | `69.94% <100.00%> (+2.37%)` | :arrow_up: | | [src/Operators/banded/Conversion.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9iYW5kZWQvQ29udmVyc2lvbi5qbA==) | `83.05% <100.00%> (+5.08%)` | :arrow_up: | | ... and [1 more](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | | ... and [48 files with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/475/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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