Fix build, sync and cleanup the manpage

By order of commit:

1. The `pod2man` command finishes with an error status because `doc/manual.pod` uses non-standard formatting to force spacing, and this causes `make` to fail. This commit has `pod2man` run with `--errors=none`, which tells it not to produce any errors, and the build now finishes successfully.
2. The commands `task merge` and `task push` are deprecated in the current version of **Taskwarrior (2.3.0)**, and future releases will apparently remove them altogether. This commit replaces those commands with the `task sync` command that replaces them.
3. I noticed a few inconsistencies in the way the man page was formatted and tried to smooth some of them over in this commit.
4. This commit updates the manpage to reflect the change from `task merge && task push` to `task sync`.

Cheers!