Enhance nanorc man page wording

Hello Benno,

Lately, I was creating nano syntax highlighting files when I encountered a
strange behavior. When I wrote:

syntax "test" "\.my_config$"

It correctly highlights the file /home/.my_config , but then failed when I
added an anchor to it:

syntax "test" "^\.my_config$"

I looked up the man page over and over again, but it stated clearly
<https://www.manpagez.com/man/5/nanorc/nanorc-6.4.php>that:

When *nano *is run, this syntax will be automatically activated if the
> current *filename* matches the extended regular expression *fileregex*.


which means that it's the *filename* that matches the regex. In convention
<https://en.wikipedia.org/wiki/Filename>, *filename* is the basename
including the extension, which is not the same as the full path name. This
is where I found frustrated, as nano is in fact matching the full pathname
instead! Therefore, I strongly suggest clarifying what *filename *
actually means.

Thanks,
davidhcefx