This option is used in several places, and deprecating it causes
warnings.
Because change.api.excludeMergeableInChangeInfo is configurable, callers
have no way to reliably request a change without computing mergeability
without specifying the SKIP_MERGEABLE option.
Remove the @Deprecated annotation, and clarify the documentation.
Change-Id: I20fdadbda12f3d7e434d2f61f5a93569a0df3674
ec1769201b