update introductory paragraphs to remove any indication that plone.api will go away.

give indication of the versioning policy when discussing the stability of the package

minor spelling fix
omit needless words

clean up the sentence structure a bit.
clean up and consolidate english in about.rst
fixing this formatted string block.  It still fails to appear due to odd css.  we need to review that
fixing the english for this section of the docs