web: Fix accidental additon of refract() prototypes and update README.

README.md

@@ -155,17 +155,24 @@ changes are quite infrequent. For windows you can get binaries from
 The command to rebuild is:
 
 ```bash
+m4 -P MachineIndependent/glslang.m4 > MachineIndependent/glslang.y
 bison --defines=MachineIndependent/glslang_tab.cpp.h
       -t MachineIndependent/glslang.y
       -o MachineIndependent/glslang_tab.cpp
 ```
 
-The above command is also available in the bash script at
-`glslang/updateGrammar`.
+The above commands are also available in the bash script in `updateGrammar`,
+when executed from the glslang subdirectory of the glslang repository.
+With no arguments it builds the full grammar, and with a "web" argument,
+the web grammar subset (see more about the web subset in the next section).
 
 ### WASM for the the Web
 
 Use the steps in [Build Steps](#build-steps), which following notes/exceptions:
+* For building the web subset of core glslang:
+  + `m4` also needs a `-DGLSLANG_WEB` argument, or simply execute `updateGrammar web` from the glslang subdirectory
+  + turn off the CMAKE options for `BUILD_TESTING`, `ENABLE_OPT`, and `INSTALL_GTEST`,
+    while turning on `ENABLE_GLSLANG_WEB`
 * `emsdk` needs to be present in your executable search path, *PATH* for
   Bash-like enivironments
   + Instructions located