perlretut.pod: Rephrase to be consistent with other pods

This pod was calling bracketed character classes as just plain
"character classes", but in one place it referred to the period as a
character class as well, which is the terminology used elsewhere.  This
commit notes the distinction.
perlretut.pod: Rephrase about \p{}.

This is in response to ticket [perl #118667].  This commit removes the
confusing table of equivalent Unicode properties.  It contained material
about Unicode without adequate explanation beyond what a tutorial reader
would be expected to know, so I just pulled it out.  The POSIX classes
haven't been introduced at this point, which really are needed for
understanding this.  Below, where they are introduced, I believe the
examples make things adequately clear.