man9: Add a manual page for filedesc(9)

[raghav.original_gmail.com]

Summary

man9: Add a manual page for filedesc(9)

A reference manpage already exists in NetBSD and this follows that general structure.

I would appreciate review on the DESCRIPTION section to see if it is accurate.
for reference here is the NetBSDs manpage for filedesc(9):
https://man.freebsd.org/cgi/man.cgi?query=filedesc&apropos=0&sektion=9&manpath=NetBSD+10.1&format=html

[maxim]

Hi Raghav,

Thanks for the patch.

Several observations:

• Please run it through a spell checker, there are a couple of misspellings

• Please place each sentence on a new line: it simplify future maintenance, especially for translators

• Did you write the manpage from scratch or were any parts of it taken from elsewhere, e.g., from NetBSD? If the latter is true, the license section should reflect it. I am asking because I see that it least some wording matches NetBSD closely. Also, it may worth importing NetBSD manpage entirely, although I haven't checked if their code is identical or similar to us.

Please note that I didn't check whether the man page itself correct, matches the code, or if FreeBSD even needs it.

Maxim