Move boilerplate docs for 'if_generation_match' et al. out of method docstrings

[tseaver]

See also #455.

There are a ton of API-invoking methods which take if_generation_match, if_generation_not_match, etc. and have docstring entries for each of them which are both too verbose and too terse: they don't include / reference examples, and they are wordy. y preference, they would point instead to a narrative page (built from e.g. docs/conditional_arguments.rst) which would include comprehensive examples for them.