Recommend rtc::StringBuilder rather than +

Bug: none
Change-Id: Ib6d5d582b1c1c5032ba5c388e47963784db2b6b2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/325282
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41039}

g3doc/style-guide.md

@@ -127,6 +127,19 @@ For example,
 See the [source code for `rtc::ArrayView`](api/array_view.h) for more detailed
 docs.
 
+### Strings
+
+WebRTC uses std::string, with content assumed to be UTF-8. Note that this
+has to be verified whenever accepting external input.
+
+For concatenation of strings, use rtc::SimpleStringBuilder.
+
+The following string building tools are NOT recommended:
+* The + operator. See https://abseil.io/tips/3 for why not.
+* absl::StrCat, absl::StrAppend, absl::StrJoin. These are optimized for
+  speed, not code size, and have significant code size overhead.
+* strcat. It is too easy to create buffer overflows.
+
 ### sigslot
 
 SIGSLOT IS DEPRECATED.