[doc] missing doc about :+ vs + expansion

[Christophe Lyon]

Hi,

I think I've noticed a bit of explanation missing in the documentation
about parameter expansion.

bash supports :+ and + expansion, but 'man bash' describes the former only.
This is also true for the similar expansions (:-, :=, :?)

Some documentation is available at:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02
and
https://www.tldp.org/LDP/abs/html/parameter-substitution.html

Thanks,

Christophe