(#22643) Document the file_metadata endpoint.

This patch documents the file_metadata endpoint (a find operation)
and the file_metadatas endpoint (a search operation).  It includes a
json schema for the returned file_metadata object, and spec tests
for validating some variants of file metadata against that schema.
(#22643) Fix lists and remove non-ascii characters
(#22643) Fix a typo.
(#22643) Fix sub-bullets.

Note that the fix for the sub-bullets broke the code block immediately
following.  The workaround was to add the ` ` line.  For more:

http://meta.stackoverflow.com/questions/23822/markdown-code-block-problem
Merge pull request #1961 from kylog/issue/22643-file_metadata-api-doc

(#22643) Document the file_metadata endpoint.