By placing docs per enum value rather than in a large block,
the automated doc generation tool can make nicer docs,
as could other automated tools, such as
the mooted https://github.com/libgit2/git2go/issues/427.

The current rendering is somewhat ugly:

https://libgit2.github.com/libgit2/#HEAD/type/git_pathspec_flag_t

No textual changes, just reorganization.