env doc update for gcc_plugin
MODIFY
docs/env_variables.txt
docs/env_variables.txt
@@ -7,8 +7,8 @@ Environmental variables
users or for some types of custom fuzzing setups. See README for the general
instruction manual.
-1) Settings for afl-gcc, afl-clang, and afl-as
-----------------------------------------------
+1) Settings for afl-gcc, afl-clang, and afl-as - and gcc_plugin afl-gcc-fast
+----------------------------------------------------------------------------
Because they can't directly accept command-line options, the compile-time
tools make fairly broad use of environmental variables:

env doc update for gcc_plugin