mixer .8: Clean up the manual page

• Use correct macros (e.g., Pa for paths, Ar for arguments, Cm for command modifiers).
• Pet igor and mandoc -Tlint (e.g., start sentences after a newline).
• Use Ta instead of a tab character in tables.
• Stylize all table headers with Sy consistency.
• Add a missing "vol" variant to the synopsis of "dev.volume".
• Sort dev.recsrc command modifiers consistency.
• Use "Bd -literal" for code blocks in the examples. "Bl -tag" is not the right macro for that.

Fixes: 903873ce1560 Implement and use new mixer(3) library for FreeBSD.