[uniffi] Explain what an “application message” is

[mgeisler]

Issues:

Addresses #81.

Description of changes:

This expands the docstring a little for the ReceivedMessage::ApplicationMessage enum and the encrypt_application_message method.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.52%. Comparing base (8f7d38b) to head (4bc055f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #138   +/-   ##
=======================================
  Coverage   89.52%   89.52%           
=======================================
  Files         173      173           
  Lines       31450    31450           
=======================================
  Hits        28156    28156           
  Misses       3294     3294           

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