encoding/json: document unsupported float values

Fixes #59627

Change-Id: Icd6a9803e213596de6136ec980b0a352c450e6f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/496142
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/encoding/json/encode.go

@@ -42,6 +42,7 @@ import (
 // Boolean values encode as JSON booleans.
 //
 // Floating point, integer, and Number values encode as JSON numbers.
+// NaN and +/-Inf values will return an [UnsupportedValueError].
 //
 // String values encode as JSON strings coerced to valid UTF-8,
 // replacing invalid bytes with the Unicode replacement rune.