Small documentation feature request

Hi, Akim and bisonistas,

This came up today on SO:
https://stackoverflow.com/questions/62658368/unknown-type-yypcontext-t-in-bison.
The user complained that their attempt to declare `yyreport_syntax_error`
failed because `yypcontext_t` was not defined. (The declaration was put in
a `%{` code section in the prologue.)

That seems to be expected, since the structure is not declared until after
all the prologue code has been included, and furthermore
yyreport_syntax_error is declared as `static`, implying that it must be
defined in the bison input file in the final code section.

However, the manual does not mention this fact, and I think that it is not
immediately obvious. It would, IMHO, be a good idea to put a small note
about that in the manual chapter which describes that function.

Saludos,
Rici