bug#6175: dirname manpage and info page partially wrong/misleading

[Filipus Klutiero]

This report is about 2 separate issues in the dirname documentation. The 
info page is much better than the manual page, but says:
> `dirname' prints all but the final slash-delimited component of a
> string (presumably a file name, but also works on directories).
The parenthese is wrong; there is no reason to presume the file given is 
a regular file, dirname is equally useful on directories. "presumably" 
should perhaps be replaced by "usually", or removed.

The manual page is much more confusing, saying:

> DESCRIPTION
>
>
>  Print NAME with its trailing /component removed; if NAME contains no 
> /'s, output `.' (meaning the current directory).

dirname is more complex than just removing a trailing slash followed by 
a pathname component if that is the ending. For example, it also removes 
a slash followed by a pathname component followed by a slash, if that is 
how the path terminates. The manual should probably adopt the info 
page's formulation, which is more vague, but not wrong.