Patch #1489771: update syntax rules in Python Reference Manual.

Patch #1489771: update syntax rules in Python Reference Manual.
Python 2.5 added support for explicit relative import statements and
yield expressions, which were missing in the manual.
Also fix grammar productions that used the names from the Grammar file,
markup that broke the generated grammar.txt, and wrap some lines that
broke the pdf output.
 (backport from rev. 54559)