[PATCH] update -Wall and -Wextra documentation

[Martin Sebor <msebor@gmail.com>]

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A number of warning options are missing from the list shown under
-Wall (for example, -Winit-self, -Wlogical-not-parentheses, and
-Wplacement-new).  The patch adds the missing ones and adjusts
some of the existing ones to maintain an alphabetical order.  It
also makes a couple of minor clarifications to -Wextra, and adds
an example to the -Wplacement-new option.

Martin

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2015-12-09  Martin Sebor  <msebor@redhat.com>

	* invoke.texi (Warning Options): Update -Wall options.  Clarify
	when some -Wextra options are enabled.  Add -Wplacement-new example.

Index: invoke.texi
===================================================================
--- invoke.texi	(revision 231436)
+++ invoke.texi	(working copy)
@@ -3541,26 +3541,33 @@
 
 @gccoptlist{-Waddress   @gol
 -Warray-bounds=1 @r{(only with} @option{-O2}@r{)}  @gol
+-Wbool-compare  @gol
 -Wc++11-compat  -Wc++14-compat@gol
 -Wchar-subscripts  @gol
+-Wcomment  @gol
+-Wduplicated-cond  @gol
 -Wenum-compare @r{(in C/ObjC; this is on by default in C++)} @gol
+-Wformat   @gol
+-Wimplicit @r{(C and Objective-C only)} @gol
 -Wimplicit-int @r{(C and Objective-C only)} @gol
 -Wimplicit-function-declaration @r{(C and Objective-C only)} @gol
--Wbool-compare  @gol
--Wduplicated-cond  @gol
--Wcomment  @gol
--Wformat   @gol
+-Winit-self @r{(only for C++)} @gol
+-Wlogical-not-parentheses
 -Wmain @r{(only for C/ObjC and unless} @option{-ffreestanding}@r{)}  @gol
 -Wmaybe-uninitialized @gol
+-Wmemset-transposed-args @gol
 -Wmissing-braces @r{(only for C/ObjC)} @gol
+-Wnarrowing @r{(only for C++)}  @gol
 -Wnonnull  @gol
 -Wopenmp-simd @gol
 -Wparentheses  @gol
+-Wplacement-new @r{(only for C++)} @gol
 -Wpointer-sign  @gol
 -Wreorder   @gol
 -Wreturn-type  @gol
 -Wsequence-point  @gol
 -Wsign-compare @r{(only in C++)}  @gol
+-Wsizeof-pointer-memaccess @gol
 -Wstrict-aliasing  @gol
 -Wstrict-overflow=1  @gol
 -Wswitch  @gol
@@ -3598,10 +3605,10 @@
 -Wmissing-parameter-type @r{(C only)}  @gol
 -Wold-style-declaration @r{(C only)}  @gol
 -Woverride-init  @gol
--Wsign-compare  @gol
+-Wsign-compare @r{(C only)} @gol
 -Wtype-limits  @gol
 -Wuninitialized  @gol
--Wshift-negative-value  @gol
+-Wshift-negative-value @r{(in C++03 and in C99 and newer)}  @gol
 -Wunused-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)} @gol
 -Wunused-but-set-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)}  @gol
 }
@@ -4895,8 +4902,15 @@
 @opindex Wno-placement-new
 Warn about placement new expressions with undefined behavior, such as
 constructing an object in a buffer that is smaller than the type of
-the object.
-
+the object.  For example, the placement new expression below is diagnosed
+because it attempts to construct an array of 64 integers in a buffer only
+64 bytes large.
+@smallexample
+char buf [64];
+new (buf) int[64];
+@end smallexample
+This warning is enabled by @option{-Wall}.
+  
 @item -Wpointer-arith
 @opindex Wpointer-arith
 @opindex Wno-pointer-arith
@@ -5113,8 +5127,9 @@
 @cindex signed and unsigned values, comparison warning
 Warn when a comparison between signed and unsigned values could produce
 an incorrect result when the signed value is converted to unsigned.
-This warning is also enabled by @option{-Wextra}; to get the other warnings
-of @option{-Wextra} without this warning, use @option{-Wextra -Wno-sign-compare}.
+In C++, this warning is also enabled by @option{-Wall}.  In C, it is also
+enabled by @option{-Wextra}; to get the other warnings of @option{-Wextra}
+without this warning, use @option{-Wextra -Wno-sign-compare}.
 
 @item -Wsign-conversion
 @opindex Wsign-conversion