fmt, strconv: document use of Unicode replacement character in %q

Fixes #51526.

Change-Id: I365a763454bd201f804df29f800416b1731b8ebc
Reviewed-on: https://go-review.googlesource.com/c/go/+/390436
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rsc

src/fmt/doc.go

@@ -110,6 +110,10 @@ For complex numbers, the width and precision apply to the two
 components independently and the result is parenthesized, so %f applied
 to 1.2+3.4i produces (1.200000+3.400000i).
 
+When formatting a single integer code point or a rune string (type []rune)
+with %q, invalid Unicode code points are changed to the Unicode replacement
+character, U+FFFD, as in strconv.QuoteRune.
+
 Other flags:
 	+	always print a sign for numeric values;
 		guarantee ASCII-only output for %q (%+q)

src/strconv/quote.go

@@ -165,6 +165,8 @@ func AppendQuoteToGraphic(dst []byte, s string) []byte {
 // QuoteRune returns a single-quoted Go character literal representing the
 // rune. The returned string uses Go escape sequences (\t, \n, \xFF, \u0100)
 // for control characters and non-printable characters as defined by IsPrint.
+// If r is not a valid Unicode code point, it is interpreted as the Unicode
+// replacement character U+FFFD.
 func QuoteRune(r rune) string {
 	return quoteRuneWith(r, '\'', false, false)
 }
@@ -179,6 +181,8 @@ func AppendQuoteRune(dst []byte, r rune) []byte {
 // the rune. The returned string uses Go escape sequences (\t, \n, \xFF,
 // \u0100) for non-ASCII characters and non-printable characters as defined
 // by IsPrint.
+// If r is not a valid Unicode code point, it is interpreted as the Unicode
+// replacement character U+FFFD.
 func QuoteRuneToASCII(r rune) string {
 	return quoteRuneWith(r, '\'', true, false)
 }
@@ -193,6 +197,8 @@ func AppendQuoteRuneToASCII(dst []byte, r rune) []byte {
 // the rune. If the rune is not a Unicode graphic character,
 // as defined by IsGraphic, the returned string will use a Go escape sequence
 // (\t, \n, \xFF, \u0100).
+// If r is not a valid Unicode code point, it is interpreted as the Unicode
+// replacement character U+FFFD.
 func QuoteRuneToGraphic(r rune) string {
 	return quoteRuneWith(r, '\'', false, true)
 }