bytes,slices,strings: ContainsFunc: document short-circuit semantics

I assume this was the intent but was not documented as an oversight.

Change-Id: I2d62b8b28ed7bca0d935788a39579b13d6503624
Reviewed-on: https://go-review.googlesource.com/c/go/+/754242
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>

src/bytes/bytes.go

@@ -89,6 +89,7 @@ func ContainsRune(b []byte, r rune) bool {
 }
 
 // ContainsFunc reports whether any of the UTF-8-encoded code points r within b satisfy f(r).
+// It stops as soon as a call to f returns true.
 func ContainsFunc(b []byte, f func(rune) bool) bool {
 	return IndexFunc(b, f) >= 0
 }

src/slices/slices.go

@@ -120,6 +120,7 @@ func Contains[S ~[]E, E comparable](s S, v E) bool {
 
 // ContainsFunc reports whether at least one
 // element e of s satisfies f(e).
+// It stops as soon as a call to f returns true.
 func ContainsFunc[S ~[]E, E any](s S, f func(E) bool) bool {
 	return IndexFunc(s, f) >= 0
 }

src/strings/strings.go

@@ -74,6 +74,7 @@ func ContainsRune(s string, r rune) bool {
 }
 
 // ContainsFunc reports whether any Unicode code points r within s satisfy f(r).
+// It stops as soon as a call to f returns true.
 func ContainsFunc(s string, f func(rune) bool) bool {
 	return IndexFunc(s, f) >= 0
 }