Docs: %adapt::sum_trs entries may well exceed %icap::tt (#914)

Docs: %adapt::sum_trs entries may well exceed %icap::tt (#914)

%icap::tt documentation incorrectly implied that the measurement
includes the entire ICAP transaction(s) lifetime. In reality, individual
ICAP transaction contribution stops with
Adaptation::Icap::ModXactLauncher::swanSong(), which is normally
triggered by Adaptation::Icap::Launcher::noteAdaptationAnswer(). Here,
the "answer" does not include the entire ICAP response, but just enough
information to form adapted HTTP message headers (echoed or received).
Thus, a large or slow ICAP response body may result in %adapt::sum_trs
values that far exceed the corresponding %icap::tt "total".

This change does not imply that %icap::tt should (not) work differently.

Also fixed a typo in %adapt::all_trs and polished %adapt::sum_trs docs.