Merge "evemu-record: Document the --timestamp-base=epoch option" into main

### Timestamp bases

By default, event timestamps are recorded relative to the time of the first event received during

the recording. Passing `--timestamp-base=boot` causes the timestamps to be recorded relative to the

system boot time instead. While this does not affect the playback of the recording, it can be useful

for matching recorded events with other logs that use such timestamps, such as `dmesg` or the

touchpad gesture debug logs emitted by `TouchpadInputMapper`.

the recording. Passing `--timestamp-base=epoch` causes the timestamps to be recorded as Unix

timestamps, relative to the Unix epoch (00:00:00 UTC on 1st January 1970). While this does not

affect the playback of the recording, it can make the events in the recording easier to match up

with those from other log sources, like logcat.

[FreeDesktop]: https://gitlab.freedesktop.org/libevdev/evemu

[format]: https://gitlab.freedesktop.org/libevdev/evemu#device-description-format