Consolidate extended retry docstring information in docs file, with links

[andrewsg]

Method arguments accepting Retry and ConditionalRetryPolicy have long, detailed docstrings that may be better consolidated in the actual documentation directory and served via a link in the docstring.

[tseaver]

Duplicate of #455? That one was closed in #461.

[cojenco]

This seems to be closed via #461. @andrewsg please reopen if there are other action items.