docs: improve checkpoint description

[njzjz]

Fix #2767.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (b38a8fc) 78.67% compared to head (eadc67e) 78.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2784      +/-   ##
==========================================
- Coverage   78.67%   78.66%   -0.01%     
==========================================
  Files         244      244              
  Lines       25813    25813              
  Branches     1519     1519              
==========================================
- Hits        20308    20306       -2     
- Misses       5114     5116       +2     
  Partials      391      391              

Files Changed	Coverage Δ
deepmd/entrypoints/train.py	87.66% <ø> (ø)
deepmd/model/frozen.py	80.39% <ø> (ø)
deepmd/model/linear.py	75.94% <ø> (ø)
deepmd/model/model.py	75.55% <ø> (ø)
deepmd_cli/main.py	100.00% <ø> (ø)
deepmd/utils/argcheck.py	96.05% <100.00%> (ø)

... and 2 files with indirect coverage changes

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