pw_string: Docs for choosing between InlineString and StringBuilder
pw_string: Docs for choosing between InlineString and StringBuilder

- Add a section about how to choose between InlineString and
  StringBuilder.
- Emphasize that InlineString can be used to add null-terminators to
  length-delimited strings (std::string_view).

Change-Id: Ie45de936afc48dbbd134335d9a9765a8501e4789
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/109230
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed