fmt: document that Scan etc. accept 'p' format floats

In the Scan functions documentation, clarify that for float/complex
literals in scientific notation both decimal (e) and binary (p)
exponents are accepted.

Fixes #24453

Change-Id: Ic6dcdb0c36e088ffb65177038aff7a57ab56b805
Reviewed-on: https://go-review.googlesource.com/107416Reviewed-by: Rob Pike <r@golang.org>

src/fmt/doc.go

@@ -281,9 +281,11 @@
 	The verbs behave analogously to those of Printf.
 	For example, %x will scan an integer as a hexadecimal number,
 	and %v will scan the default representation format for the value.
-	The Printf verbs %p and %T and the flags # and + are not implemented,
-	and the verbs %e %E %f %F %g and %G are all equivalent and scan any
-	floating-point or complex value.
+	The Printf verbs %p and %T and the flags # and + are not implemented.
+	The verbs %e %E %f %F %g and %G are all equivalent and scan any
+	floating-point or complex value. For float and complex literals in
+	scientific notation, both the decimal (e) and binary (p) exponent
+	formats are supported (for example: "2.3e+7" and "4.5p-8").
 
 	Input processed by verbs is implicitly space-delimited: the
 	implementation of every verb except %c starts by discarding