doc: various update to command documents

1. consistent usage section (go tool xxx)
        2. reformat cmd/ld document with minor correction
           document which -H flags are valid on which ld
           document -d flag can't be used on Windows.
           document -Hwindowsgui

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/5782043

src/cmd/gc/doc.go

@@ -26,7 +26,7 @@ package P to read the files of P's dependencies, only the compiled output
 of P.
 
 Usage:
-	6g [flags] file...
+	go tool 6g [flags] file...
 The specified files must be Go source files and all part of the same package.
 Substitute 6g with 8g or 5g where appropriate.
 

src/cmd/ld/doc.go

@@ -9,45 +9,52 @@ Ld is the portable code for a modified version of the Plan 9 linker.  The origin
 	http://plan9.bell-labs.com/magic/man2html/1/2l
 
 It reads object files (.5, .6, or .8 files) and writes a binary named for the
-architecture (5.out, 6.out, 8.out) by default.
+architecture (5.out, 6.out, 8.out) by default (if $GOOS is windows, a .exe suffix
+will be appended).
 
 Major changes include:
-	- support for ELF and Mach-O binary files
+	- support for ELF, Mach-O and PE binary files
 	- support for segmented stacks (this feature is implemented here, not in the compilers).
 
 Original options are listed on the manual page linked above.
 
+Usage:
+	go tool 6l [flags] mainObj
+Substitute 6l with 8l or 5l as appropriate.
+
 Options new in this version:
 
--d
-	Elide the dynamic linking header.  With this option, the binary
-	is statically linked and does not refer to dynld.  Without this option
-	(the default), the binary's contents are identical but it is loaded with dynld.
--Hdarwin
-	Write Apple Mach-O binaries (default when $GOOS is darwin)
--Hlinux
-	Write Linux ELF binaries (default when $GOOS is linux)
--Hfreebsd
-	Write FreeBSD ELF binaries (default when $GOOS is freebsd)
--Hnetbsd
-	Write NetBSD ELF binaries (default when $GOOS is netbsd)
--Hopenbsd
-	Write OpenBSD ELF binaries (default when $GOOS is openbsd)
--Hwindows
-	Write Windows PE32+ binaries (default when $GOOS is windows)
--I interpreter
-	Set the ELF dynamic linker to use.
--L dir1 -L dir2
-	Search for libraries (package files) in dir1, dir2, etc.
-	The default is the single location $GOROOT/pkg/$GOOS_amd64.
--r dir1:dir2:...
-	Set the dynamic linker search path when using ELF.
--V
-	Print the linker version.
--X symbol value
-	Set the value of an otherwise uninitialized string variable.
-	The symbol name should be of the form importpath.name,
-	as displayed in the symbol table printed by "go tool nm".
-
+	-d
+		Elide the dynamic linking header.  With this option, the binary
+		is statically linked and does not refer to a dynamic linker.  Without this option
+		(the default), the binary's contents are identical but it is loaded with a dynamic
+		linker. This flag cannot be used when $GOOS is windows.
+	-Hdarwin     (only in 6l/8l)
+		Write Apple Mach-O binaries (default when $GOOS is darwin)
+	-Hlinux
+		Write Linux ELF binaries (default when $GOOS is linux)
+	-Hfreebsd    (only in 6l/8l)
+		Write FreeBSD ELF binaries (default when $GOOS is freebsd)
+	-Hnetbsd     (only in 6l/8l)
+		Write NetBSD ELF binaries (default when $GOOS is netbsd)
+	-Hopenbsd    (only in 6l/8l)
+		Write OpenBSD ELF binaries (default when $GOOS is openbsd)
+	-Hwindows    (only in 6l/8l)
+		Write Windows PE32+ Console binaries (default when $GOOS is windows)
+	-Hwindowsgui (only in 6l/8l)
+		Write Windows PE32+ GUI binaries
+	-I interpreter
+		Set the ELF dynamic linker to use.
+	-L dir1 -L dir2
+		Search for libraries (package files) in dir1, dir2, etc.
+		The default is the single location $GOROOT/pkg/$GOOS_$GOARCH.
+	-r dir1:dir2:...
+		Set the dynamic linker search path when using ELF.
+	-V
+		Print the linker version.
+	-X symbol value
+		Set the value of an otherwise uninitialized string variable.
+		The symbol name should be of the form importpath.name,
+		as displayed in the symbol table printed by "go tool nm".
 */
 package documentation

src/cmd/nm/doc.go

@@ -14,7 +14,8 @@ Plan 9 C compiler.
 This implementation adds the flag -S, which prints each symbol's size 
 in decimal after its address.
 
-It is installed as go tool nm and is architecture-independent.
+Usage:
+	go tool nm [-aghnsTu] file
 
 */
 package documentation