Documentation: Remove leftover mentions of background threads

[tatsuya6502]

Fixes #464.

• Remove leftover mentions of background threads from invalidate_all and invalidate_entries_if methods of all cache implementations.
• Add the following chapters to the crate top level doc page:
  • Cache Policies
  • Implementation Details, with explanations about the channels and maintenance tasks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.13%. Comparing base (9d78c2c) to head (cd2b3b3).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
- Coverage   94.24%   94.13%   -0.11%     
==========================================
  Files          44       44              
  Lines       20559    20559              
==========================================
- Hits        19376    19354      -22     
- Misses       1183     1205      +22     

[tatsuya6502]

Merging.