[Docs] Doc about using CombinedDataset

[Ben-Louis]

Motivation

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Patch coverage: 81.59% and project coverage change: -0.05 ⚠️

Comparison is base (2016609) 82.21% compared to head (e2e353a) 82.16%.

❗ Current head e2e353a differs from pull request most recent head cf5e82f. Consider uploading reports for the commit cf5e82f to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2025      +/-   ##
===========================================
- Coverage    82.21%   82.16%   -0.05%     
===========================================
  Files          220      224       +4     
  Lines        13100    13362     +262     
  Branches      2228     2265      +37     
===========================================
+ Hits         10770    10979     +209     
- Misses        1832     1870      +38     
- Partials       498      513      +15     

Flag	Coverage Δ
unittests	82.16% <81.59%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmpose/datasets/samplers.py	27.58% <0.00%> (-1.51%)	⬇️
mmpose/models/heads/heatmap_heads/__init__.py	100.00% <ø> (ø)
mmpose/models/losses/loss_wrappers.py	100.00% <ø> (ø)
mmpose/models/necks/gap_neck.py	59.09% <ø> (ø)
mmpose/models/utils/transformer.py	82.40% <ø> (ø)
mmpose/visualization/local_visualizer.py	79.00% <40.00%> (-0.66%)	⬇️
mmpose/engine/hooks/ema_hook.py	68.75% <68.75%> (ø)
mmpose/models/utils/rtmcc_block.py	72.65% <72.65%> (ø)
mmpose/models/heads/coord_cls_heads/rtmcc_head.py	96.66% <96.66%> (ø)
mmpose/datasets/dataset_wrappers.py	93.61% <100.00%> (ø)
... and 8 more

... and 2 files with indirect coverage changes

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