add description to sccache-dist commands

[iTrooz]

Very simple PR, self-explaining title

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.96%. Comparing base (86abf90) to head (6d30ab9).

Files with missing lines	Patch %	Lines
src/bin/sccache-dist/cmdline/parse.rs	80.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2413      +/-   ##
==========================================
- Coverage   70.96%   70.96%   -0.01%     
==========================================
  Files          65       65              
  Lines       35523    35535      +12     
==========================================
+ Hits        25209    25217       +8     
- Misses      10314    10318       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.