Clarify GCC/Clang compiler flags

Added a description for `-pedantic` and clarified similarly to the MSVC section

Author: JonnyPtn

02-Use_the_Tools_Available.md

@@ -101,8 +101,9 @@ You should use as many compilers as you can for your platform(s). Each compiler
 
 ### GCC / Clang
 
-`-Wall -Wextra -Wshadow -Wnon-virtual-dtor -pedantic`
+`-Wall -Wextra -Wshadow -Wnon-virtual-dtor -pedantic` - use these and consider the following (see descriptions below)
 
+ * `-pedantic` - Warn on language extensions
  * `-Wall -Wextra` reasonable and standard
  * `-Wshadow` warn the user if a variable declaration shadows one from a parent context
  * `-Wnon-virtual-dtor` warn the user if a class with virtual functions has a non-virtual destructor. This helps catch hard to track down memory errors