Manual: clarify what POSIX stands for

[Alan Urmancheev]

Hi.

Currently, Bash's manual definitions section mentions POSIX, but doesn't
explain what that abbreviature stands for («portable operating system
interface X», or something like that).

I think that abbreviatures can be confusing, especially when you don't get
to know what they stand for.

Therefore, I propose to add the meaning of the abbreviature to the manual.

Thanks!

The referenced part:
https://www.gnu.org/software/bash/manual/bash.html#Definitions

With respect,
Alan Urmancheev