verify-grammar path to rust.md

When calling

   make verify-grammar

rust.md cannot be found, because the
path to rust.md is missing.

The path is set to:

   $(D)/rust.md

This can only be tested, when llnextgen is installed.

Signed-off-by: Jan Kobler <eng1@koblersystems.de>
verify-grammar keyword crate

When calling

   make verify-grammar

or when llnextgen is not installed:

   python2.7 src/etc/extract_grammar.py <src/doc/rust.md

an error is reported by extract_grammar.py that the

keyword "crate" is not defined.

This patch adds the keyword "crate" to the grammar in rust.md.

Signed-off-by: Jan Kobler <eng1@koblersystems.de>
verify-grammar llnextgen

When calling

    make verify-grammar

a lot of errors are reported by llnextgen.

Only simple errors like:

  missing semicolons,
  missing single quotes,
  usage of parentheses instead of squared brackets or
  usage of single quote instead of double quote

are fixed by this patch.

This can only be tested, when llnextgen is installed.

Signed-off-by: Jan Kobler <eng1@koblersystems.de>
auto merge of #13168 : jankobler/rust/verify-grammar-02, r=brson

This fixes some problems  with

     make verify-grammar

llnextgen still reports a lot of errors

FYI: My build directory /my-test/build is different from the source directory /my-test/rust.
cd  /my-test/build
/my-test/rust/configure --prefix=/my-test/bin
make
make install
make verify-grammar