Docs: clarify getSharedPreferences behavior

No change to logic, only documentation.

This updates Context#getSharedPreferences docs:

 * Fix a typo ("thead" -> "thread")
 * Formatting: use <p> when appropriate, {@link} to other APIs, limit
   the @param to only a single sentence
 * Mention getSharedPreferences() may actually perform disk IO (this was
   undocumented)
 * Mention Editor#apply() as well as #commit()

Fixes: 140137651
Test: make docs
Change-Id: I245371d13d4f317c6c522debcdf7938ab783f897