builtin: make len's godoc less ambiguous

The len godoc uses a blockquote to list the rules for its semantics.

The item that describes channels is a bit long, so it's split in two
lines. However, the first line ends with a semicolon, and the second
line can be read as a sentence of its own, so it's easy to misinterpret
that the two lines are separate.

Making that easy mistake would lead to an incorrect understanding of
len:

	if v is nil, len(v) is zero.

This could lead us to think that len(nil) is valid and should return
zero. When in fact, that statement only applies to nil channels.

To make this less ambiguous, add a bit of indentation to the follow-up
line, to align with the channel body. If lists are added to godoc in the
future via #7873, perhaps this text can be simplified.

Fixes #30349.

Change-Id: I84226edc812d429493137bcc65c332e92d4e6c87
Reviewed-on: https://go-review.googlesource.com/c/go/+/167403
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/builtin/builtin.go

@@ -151,7 +151,7 @@ func delete(m map[Type]Type1, key Type)
 //	Slice, or map: the number of elements in v; if v is nil, len(v) is zero.
 //	String: the number of bytes in v.
 //	Channel: the number of elements queued (unread) in the channel buffer;
-//	if v is nil, len(v) is zero.
+//	         if v is nil, len(v) is zero.
 // For some arguments, such as a string literal or a simple array expression, the
 // result can be a constant. See the Go language specification's "Length and
 // capacity" section for details.