logcat: improve --help and error messages

Makes slight improvements to the --help text:
- Fix whitespace formatting and punctuation.
- Divide options into categories
- Clarify how the logd control functions work.
- Clarify --print.

Add additional text to the EOF error message that is returned when
logd disconnects the logcat instance, to clarify that this is intended
behavior if the logcat instance is reading too slowly.

Bug: 144311420
Test: view these messages
Change-Id: Icc12d278a8e8cb483918c0637b6fb529949cc33f