[RFD] Place to document magic pathspecs like ":/" and pathspec handling

Hello,

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I have noticed that the magic pathspec ":/" is described only in RelNotes for revision 1.7.6:

|* A magic pathspec ":/" tells a command that limits its operation to the current directory when ran from a subdirectory to work on the entire working tree. In general, ":/path/to/file" would be relative to the root of the working tree hierarchy. After "git reset --hard; edit Makefile; cd t/", "git add -u" would be a no-op, but "git add -u :/" would add the updated contents of the Makefile at the top level. If you want to name a path in the current subdirectory whose unusual name begins with ":/", you can name it by "./:/that/path" or by "\:/that/path".|

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|I think the reason might be that there was no good place to put that
information in.  Nowadays we have gitcli(7) manual page, but perhaps
it would be better to create a separate manpage for issues related
to pathspec handling (of which ":/" is only one part)... but then
what should it be named?

What do you think?