fmt: document the behaviour of %p on slices

Fixes #23291.

Change-Id: I9bb3b8246968693871d4800e2cdb2c8390f4b6a6
GitHub-Last-Rev: bc34442
GitHub-Pull-Request: #24600
Reviewed-on: https://go-review.googlesource.com/103516
Reviewed-by: Rob Pike <[email protected]>

Author: mewmew

src/fmt/doc.go

@@ -45,6 +45,8 @@
 		%q	a double-quoted string safely escaped with Go syntax
 		%x	base 16, lower-case, two characters per byte
 		%X	base 16, upper-case, two characters per byte
+	Slice:
+		%p	address of 0th element in base 16 notation, with leading 0x
 	Pointer:
 		%p	base 16 notation, with leading 0x
 		The %b, %d, %o, %x and %X verbs also work with pointers,