getopt: document special symbols that should not be used as option characters

getopt: document special symbols that should not be used as option characters

getopt(3) routines return `?` or `:` when an unknown option character is encountered
or an option is missing its required argument, respectively. It also disallows
`;` as an option character. Documenting this makes users aware that they may not use
these reserved symbols as option characters, e.g. `-?` which is used in some older programs.

Addresses: #2995
Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>