devstat(9): Update the man page to reflect the current implementation

• Rename devstat_add_entry to devstat_new_entry
• Update the description of devstat_trans_flags
• Add manpage aliases for devstat_start_transaction_bio and devstat_end_transaction_bio

PR: 157316
Submitted by: novel
Reviewed by: cem, bcr (mentor)
Approved by: bcr (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25677