libm: Correct some typos in source code comments

• s/modfied/modified/
• s/minimun/minimum/

While here, fix some mandoc warnings:

• whitespace at end of input line
• unusual Xr punctuation
• missing comma before name

Obtained from: NetBSD

(cherry picked from commit a52f4499aec707dbe8f4f40aba1ca24ab7595834)