Update documentation about tm_p.h

From-SVN: r178618

gcc/ChangeLog

+2011-09-06  Michael Meissner  <meissner@linux.vnet.ibm.com>
+
+	* doc/configfiles.texi (Configuration Files): Update documentation
+	about tm_p.h and remove FIXME comment.
+
 2011-09-06  Uros Bizjak  <ubizjak@gmail.com>
 
 	* config/i386/i386.c (ix86_function_value_regno_p): Use AX_REG.

gcc/doc/configfiles.texi

@@ -59,6 +59,14 @@ these include the autoconfigured headers generated by
 machine.
 @item
 @file{tm_p.h}, which includes the header @file{@var{machine}-protos.h}
-that contains prototypes for functions in the target @file{.c} file.
-FIXME: why is such a separate header necessary?
+that contains prototypes for functions in the target
+@file{@var{machine}.c} file.  The header @file{@var{machine}-protos.h}
+can include prototypes of functions that use rtl and tree data
+structures inside appropriate @code{#ifdef RTX_CODE} and @code{#ifdef
+TREE_CODE} conditional code segements.  The
+@file{@var{machine}-protos.h} is included after the @file{rtl.h}
+and/or @file{tree.h} would have been included.  The @file{tm_p.h} also
+includes the header @file{tm-preds.h} which is generated by
+@file{genpreds} program during the build to define the declarations
+and inline functions for the predicate functions.
 @end itemize