Doc: document bpchar, clarify relationship of text and varchar.

For some reason the "bpchar" type name was defined nowhere in
our SGML docs, although several places refer to it in passing.
Give it a proper mention under Character Types.

While here, also provide an explanation of how the text and varchar
types relate.  The previous wording seemed to be doing its best
to sweep text under the rug, which doesn't seem very appropriate
given its prominence in other parts of the docs.

Minor rearrangements and word-smithing for clarity, too.

Laurenz Albe and Tom Lane, per gripe from Yanliang Lei

Discussion: https://postgr.es/m/120b3084.56b6.1833b5ffe4b.Coremail.msdnchina@163.com

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