doc: clean up naming of various Bison files.

doc: clean up naming of various Bison files.

The Bison manual's names for various files associated with a Bison
parser has devolved into inconsistency.  This patch makes the
naming consistent for the most important files.  First, it chooses
"grammar file" over "input file".  The former appears to be more
traditional in the Bison manual, and Bison has other input
files (skeletons).  Second, it chooses "parser implementation
file" over names like "parser file", "parser source file", "parser
source code file", and "parser output file".  The new name makes
it clearer where Bison generates the main parser implementation,
and it is easily distinguishable from "parser header file".
* doc/bison.texinfo: Implement throughout.