Discourage the use of LIBC_PRINT_EXTENSION

See the discussion in apache#13536

Author: yamt

libs/libc/stdio/Kconfig

@@ -110,6 +110,17 @@ config LIBC_PRINT_EXTENSION
 	bool
 	default n
 	---help---
-		Enables vsprintf supports using "%p*" to print.
+		Enables non-standard "%p*" extensions for the printf family.
+
+		CAVEAT: If your system is using any of the printf C family
+		functions, it's probably unsafe to enable this option because
+		the extension is not compatible with the C standard. Namely,
+		an ordinary character following a "%p" conversion is interpreted
+		in a non-standard way, which might crash the code which expects
+		the standard semantics. For that reason, in the future versions
+		of NuttX, this extension might be removed, or changed in an
+		API-incompatible way to avoid conflicts with the standards.
+		(Eg. use different characters for converions specifiers,
+		or switch to a completely separate API, say "nuttx_printf".)
 
 endmenu #Standard C I/O