Describe disabling preallocated symbols in the doc.

doc/guides/compile.md

@@ -226,6 +226,15 @@ conf.exts do |exts|
 end
 ```
 
+### Preallocated Symbols
+
+By far, preallocate symbols are highly compatible with the previous versions, so
+we expect you won't see any problem with them. But just in case you face any
+issue, you can disable preallocated symbols by specifying `conf.disable_presym`.
+
+In the build process, `mrbc` under cross compiling environment will be compiled
+with this configuration.
+
 ### Mrbgems
 
 `mruby` comes with the (sort of) packaging system named `mrbgems`. To

doc/guides/symbol.md

@@ -76,3 +76,8 @@ these macros are expanded to `mrb_intern_lit` calls, therefore the mruby state
 variable is required. The above macros assume the variable name is `mrb`.  If
 its name is not `mrb`, you need to use macros with `_2` suffix, such as
 `MRB_SYM_2` to specify `mrb_state*` variable.
+
+### Disabling Preallocated Symbols
+
+By defining `MRB_NO_PRESYM` macro, you can disable preallocated symbols.
+Or you can specify `conf.disable_presym` in the configuration file.

doc/mruby3.md

@@ -90,6 +90,10 @@ changed from previous versions.
 Call `malloc_trim(0)` from mrb_full_gc() if this macro is defined.
 If you are using glibc malloc, this macro could reduce memory consumption.
 
+## `MRB_NO_PRESYM`
+
+Remove preallocated symbol system.
+
 # Command Line Program
 
 ## `bin/mruby` (by mrbgems/mruby-bin-mruby)
@@ -156,7 +160,7 @@ Jump addresses used to be specified by absolute offset from the start of `iseq`.
 
 For better and faster random number generation.
 
-## Preallocated Symnol
+## Preallocated Symbol
 
 Preallocated symbols are interned at compile-time. They can be accessed via symbols macros (e.g. `MRB_SYM()`).