RFC: Doc update for attribute

[David Wohlferd <dw at LimeGreenSocks dot com>]

After updating gcc's docs about inline asm, I'm trying to improve some 
of the related sections.  One that I feel has problems with clarity is  
__attribute__ naked.

I have attached my proposed update.  Comments/corrections are welcome.

In a related question:

To better understand how this attribute is used, I looked at the Linux 
kernel.  While the existing docs say "only ... asm statements that do 
not have operands" can safely be used, Linux routinely uses asm WITH 
operands.  Some examples:

memory clobber operand: 
http://lxr.free-electrons.com/source/arch/arm/kernel/kprobes.c#L377
Input arguments: 
http://lxr.free-electrons.com/source/arch/arm/mm/copypage-feroceon.c#L17

Since I don't know why "asm with operands" was excluded from the 
existing docs, I'm not sure whether what Linux does here is supported or 
not (maybe with some limitations?).  If someone can clarify, I'll add it 
to this text.

Even without discussing "asm with operands," I believe this text is an 
improvement.

Thanks in advance,
dw

Index: extend.texi
===================================================================
--- extend.texi	(revision 210349)
+++ extend.texi	(working copy)
@@ -3330,16 +3330,15 @@
 
 @item naked
 @cindex function without a prologue/epilogue code
-Use this attribute on the ARM, AVR, MCORE, MSP430, NDS32, RL78, RX and SPU
-ports to indicate that the specified function does not need prologue/epilogue
-sequences generated by the compiler.
-It is up to the programmer to provide these sequences. The
-only statements that can be safely included in naked functions are
-@code{asm} statements that do not have operands.  All other statements,
-including declarations of local variables, @code{if} statements, and so
-forth, should be avoided.  Naked functions should be used to implement the
-body of an assembly function, while allowing the compiler to construct
-the requisite function declaration for the assembler.
+This attribute is available on the ARM, AVR, MCORE, MSP430, NDS32, RL78, RX 
+and SPU ports.  It allows the compiler to construct the requisite function 
+declaration, while allowing the body of the function to be assembly code. 
+The specified function will not have prologue/epilogue sequences generated 
+by the compiler; it is up to the programmer to provide these sequences if 
+the function requires them. The expectation is that only Basic @code{asm} 
+statements will be included in naked functions (@pxref{Basic Asm}). While it 
+is discouraged, it is possible to write your own prologue/epilogue code 
+using asm and use ``C'' code in the middle.
 
 @item near
 @cindex functions that do not handle memory bank switching on 68HC11/68HC12