Fix docstring for `value_and_pullback_function`

[gdalle]

Fix the docstring of value_and_pullback_function to solve #124 and #119

Clarify that

• value_and_pushforward_function means (value and pushforward) function
• value_and_pullback_function means value and (pullback function)

Name some anonymous functions in the process to make the code more readable

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (211b675) 82.94% compared to head (5ef52c2) 82.55%.

Files	Patch %	Lines
src/AbstractDifferentiation.jl	88.88%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #125 +/- ## ========================================== - Coverage 82.94% 82.55% -0.39% ========================================== Files 8 8 Lines 428 430 +2 ========================================== Hits 355 355 - Misses 73 75 +2 ```

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