[release-branch.go1.20] html/template,mime/multipart: document new GO…

…DEBUG settings

This change documents the new GODEBUG settings introduced for
html/template and mime/multipart, released with Go 1.19.8 and Go 1.20.3
as part of a security fix.

Updates #59153.
For #59270.
Updates #59234.
For #59272.

Change-Id: I25f4d8245da3301dccccfb44da8ff1a5985392a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/482555
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

src/html/template/doc.go

@@ -231,5 +231,12 @@ Least Surprise Property:
 "A developer (or code reviewer) familiar with HTML, CSS, and JavaScript, who
 knows that contextual autoescaping happens should be able to look at a {{.}}
 and correctly infer what sanitization happens."
+
+As a consequence of the Least Surprise Property, template actions within an
+ECMAScript 6 template literal are disabled by default.
+Handling string interpolation within these literals is rather complex resulting
+in no clear safe way to support it.
+To re-enable template actions within ECMAScript 6 template literals, use the
+GODEBUG=jstmpllitinterp=1 environment variable.
 */
 package template

src/mime/multipart/multipart.go

@@ -9,6 +9,21 @@ Package multipart implements MIME multipart parsing, as defined in RFC
 
 The implementation is sufficient for HTTP (RFC 2388) and the multipart
 bodies generated by popular browsers.
+
+# Limits
+
+To protect against malicious inputs, this package sets limits on the size
+of the MIME data it processes.
+
+Reader.NextPart and Reader.NextRawPart limit the number of headers in a
+part to 10000 and Reader.ReadForm limits the total number of headers in all
+FileHeaders to 10000.
+These limits may be adjusted with the GODEBUG=multipartmaxheaders=<values>
+setting.
+
+Reader.ReadForm further limits the number of parts in a form to 1000.
+This limit may be adjusted with the GODEBUG=multipartmaxparts=<value>
+setting.
 */
 package multipart