Don't run lexer tests by default

mk/tests.mk

@@ -171,7 +171,7 @@ endif
 # Main test targets
 ######################################################################
 
-check: cleantmptestlogs cleantestlibs check-notidy tidy check-syntax
+check: cleantmptestlogs cleantestlibs check-notidy tidy
 
 check-notidy: cleantmptestlogs cleantestlibs all check-stage2
 	$(Q)$(CFG_PYTHON) $(S)src/etc/check-summary.py tmp/*.log

src/grammar/README.md

 Reference grammar.
 
 Uses [antlr4](http://www.antlr.org/) and a custom Rust tool to compare
-ASTs/token streams generated.
+ASTs/token streams generated. You can use the `check-syntax` make target to
+run all of the available tests.
 
-To use:
+To use manually:
 
 ```
 antlr4 RustLexer.g4