strconv, fmt: clarify behavior of CanBackquote and "%#q".

Fixes #4858.

R=golang-dev, bradfitz, r, rsc
CC=golang-dev
https://golang.org/cl/7387044

src/pkg/fmt/doc.go

@@ -74,7 +74,8 @@
 		-	pad with spaces on the right rather than the left (left-justify the field)
 		#	alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
 			0X for hex (%#X); suppress 0x for %p (%#p);
-			print a raw (backquoted) string if possible for %q (%#q);
+			for %q, print a raw (backquoted) string if strconv.CanBackquote
+			returns true;
 			write e.g. U+0078 'x' if the character is printable for %U (%#U).
 		' '	(space) leave a space for elided sign in numbers (% d);
 			put spaces between bytes printing strings or slices in hex (% x, % X)

src/pkg/strconv/quote.go

@@ -139,8 +139,9 @@ func AppendQuoteRuneToASCII(dst []byte, r rune) []byte {
 	return append(dst, QuoteRuneToASCII(r)...)
 }
 
-// CanBackquote returns whether the string s would be
-// a valid Go string literal if enclosed in backquotes.
+// CanBackquote reports whether the string s can be represented
+// unchanged as a single-line backquoted string without control
+// characters other than space and tab.
 func CanBackquote(s string) bool {
 	for i := 0; i < len(s); i++ {
 		if (s[i] < ' ' && s[i] != '\t') || s[i] == '`' {