add a few docstrings

[putianyi889]

for (l|r)(diffbc|dirichlet|neumann), ivp, bvp and periodic

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4d8c6fc) 75.28% compared to head (ee9817d) 75.26%. Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #613 +/- ## ========================================== - Coverage 75.28% 75.26% -0.02% ========================================== Files 80 80 Lines 8435 8435 ========================================== - Hits 6350 6349 -1 - Misses 2085 2086 +1 ``` | [Files](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/613?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Operators/functionals/Evaluation.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/613?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9mdW5jdGlvbmFscy9FdmFsdWF0aW9uLmps) | `88.29% <ø> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/613/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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[jishnub]

Thanks!