[eCall] Improve eCall session state documentation
[eCall] Improve eCall session state documentation

The eCall API documentation is improved to better describe the eCall
session state during a call. This helps the user understanding when he
can trigger an eCall again.

Resolves: LE-5608
Change-Id: Iaa604700cf4ceebf796a9a781ca09495429d490a
legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af