Clarify ASCII Armoring and Cleartext formats

Describe explicitly what ASCII characters are considered whitespace. Use "blank" instead of "empty" when referring to a blank line that can be either zero-length or contain only whitespace, and describe what it means. Mention that Section 7 follows the same format and restrictions of Section 6.2. Add that trailing whitespace at the end of any line is removed for both signature generation and verification.

Ref: https://mailarchive.ietf.org/arch/msg/openpgp/ryzRcpoT4peM-ebwAxy2RV05qPs