Elisp manual: mention parse-partial-sexp as the source of a parser state, etc

Elisp manual: mention parse-partial-sexp as the source of a parser state, etc

* doc/lispref/syntax.texi (Parser State): mention parse-partial-sexp as a
function returning a parser state, and the function to which one supplies a
parser state as an argument for continued parsing.
Refine the documentation of syntax-ppss-context, and correct the text
introducing it and another function.