Added some text warning that the "__" macros the <features.h>

defines internally should never be directly defined by programs.

man7/feature_test_macros.7

@@ -164,3 +164,12 @@ Other systems have an analogous file, but typically with a different name.
 This header file is automatically included by other header files as 
 required: it is not necessary to explicitly include it in order to 
 employ feature test macros.
+
+According to which of the above feature test macros are defined,
+.I <features.h>
+internally defines various other macros that are checked by 
+other glibc header files.  
+These macros have names prefixed by two underscores (e.g., __USE_MISC).
+Programs should \fInever\fP define these macros directly:
+instead, the appropriate feature test macro(s) from the 
+list above should be employed.