Include .EX/.EE in the MACRO OVERVIEW and improve its description.

Include .EX/.EE in the MACRO OVERVIEW and improve its description.

It is supported by all major man(7) implementations that G. Branden
Robinson and myself are aware of, so calling it "non-portable" can
no longer be justified.  Using it becomes increasingly more common,
so calling it "non-standard" is now misleading.  It is certainly
useful and not deprecated.

While here, also remove the word "non-standard" from the descriptions
of several other macros because it is slightly confusing.  A formal
standard for the man(7) language does not exist.  Arguably, Version 7
AT&T UNIX used to be a de-facto standard, but its influence has been
waning for 40 years, and various features that Version 7 did not
support are now widely used.