evemu-record: Document the --timestamp-base=epoch option

The timestamp base "boot" was changed to "epoch" in:
commit I80e3b4db1bcc2e6f656bfaaf7904f1678cfb29cf

Bug: 245989146
Change-Id: Id867cece5732306454d67efc58f08e0534df26f9
Flag: DOCS_ONLY
Test: None
1 file changed