Add some hopefully helpful documentation to the functions in mono_time.h

[zugz]

(I found I couldn't remember whether get_mono_time returned unix time or
not, and had to source-dive to find out. This should make that
unnecessary next time!)

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This change is 

[codecov]

Codecov Report

Merging #1239 into master will decrease coverage by 0.4%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1239     +/-   ##
========================================
- Coverage    83.4%     83%   -0.5%     
========================================
  Files          82      82             
  Lines       14685   14685             
========================================
- Hits        12251   12192     -59     
- Misses       2434    2493     +59

Impacted Files	Coverage Δ
auto_tests/toxav_many_test.c	92.5% <0%> (-4.8%)	⬇️
toxav/msi.c	64.5% <0%> (-4.2%)	⬇️
toxav/toxav.c	67.1% <0%> (-3.1%)	⬇️
toxcore/group.c	76.2% <0%> (-0.4%)	⬇️
toxcore/friend_connection.c	94.2% <0%> (-0.3%)	⬇️
toxcore/net_crypto.c	94.7% <0%> (-0.3%)	⬇️
toxcore/Messenger.c	86.9% <0%> (-0.1%)	⬇️

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[hugbubby]

adds comments
-0.5% code coverage

O.o

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained