docs(backend): improve backend module docs

[joshka]

No description provided.

[codecov]

Codecov Report

Merging #489 (f17cf92) into main (af36282) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 22.50%.

@@           Coverage Diff           @@
##             main     #489   +/-   ##
=======================================
  Coverage   90.16%   90.16%           
=======================================
  Files          41       41           
  Lines       11277    11277           
=======================================
  Hits        10168    10168           
  Misses       1109     1109           

Files Changed	Coverage Δ
src/backend/crossterm.rs	0.00% <0.00%> (ø)
src/backend/termwiz.rs	0.00% <ø> (ø)
src/backend/test.rs	94.84% <ø> (ø)
src/backend/termion.rs	26.76% <38.09%> (ø)
src/backend/mod.rs	82.92% <100.00%> (ø)

... and 2 files with indirect coverage changes