docs(man)/tags(5): write more about {tagfile}

[masatake]

@AmaiKinono:

I think we still need to write about how {tagfile} is printed in the manual, according to how it's actually implemented, so a client tool knows how to get the actual path.

I will write it in "EXCEPTIONS IN UNIVERSAL CTAGS" or somewhere in tags(5).
I would like you to review the text I will add.

Originally posted by @masatake in #3578 (comment)

[masatake]

This is the last one for 6.0.0.

[masatake]

Closed via #3599.