gc: describe debugging flags

The change to -m is the only one necessary
to close the issue.  The others are useful
to know about when debugging but shouldn't
be in the usage message since they may go
away or change at any time.

Fixes #2802.

R=lvd, rsc
CC=golang-dev
https://golang.org/cl/5606046

src/cmd/gc/lex.c

@@ -132,10 +132,25 @@ usage(void)
 {
 	print("gc: usage: %cg [flags] file.go...\n", thechar);
 	print("flags:\n");
-	// -A is allow use of "any" type, for bootstrapping
+	// -A allow use of "any" type, for bootstrapping
+	// -B disable bounds checking
+	// -E print imported declarations
+	// -K warn when lineno is zero
+	// -M print arguments to gmove
+	// -P print peephole diagnostics
+	// -R print optimizer diagnostics
+	// -g print code generation diagnostics
+	// -i print line history
+	// -j print variables to be initialized at runtime
+	// -r print generated helper functions
+	// -s print redundant types in composite literals
+	// -v print more information with -P or -R
+	// -y print declarations in cannedimports (used with -d)
+	// -% print non-static initializers
+	// -+ indicate that the runtime is being compiled
 	print("  -I DIR search for packages in DIR\n");
 	print("  -L show full path in file:line prints\n");
-	print("  -N disable optimizer\n");
+	print("  -N disable optimizations\n");
 	print("  -S print the assembly language\n");
 	print("  -V print the compiler version\n");
 	print("  -W print the parse tree after typing\n");
@@ -143,10 +158,10 @@ usage(void)
 	print("  -e no limit on number of errors printed\n");
 	print("  -f print stack frame structure\n");
 	print("  -h panic on an error\n");
-	print("  -m print about moves to heap\n");
+	print("  -l disable inlining\n");
+	print("  -m print optimization decisions\n");
 	print("  -o file specify output file\n");
 	print("  -p assumed import path for this code\n");
-	print("  -s disable escape analysis\n");
 	print("  -u disable package unsafe\n");
 	print("  -w print type checking details\n");
 	print("  -x print lex tokens\n");