DOC: tighten the comments that came with the sampler seed groups

[thc1006]

Pull request type

• ReadMe, Docs and GitHub updates

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (ruff check / ruff format --check) has passed locally
• All tests (pytest tests/unit) have passed locally

Current behavior

The comments I added in #1102 are longer than they need to be. Three of them
explain the same rule, and several narrate how the rule was arrived at instead
of stating it. seed_group in particular carries three paragraphs for a
one-line body, which is out of step with sample and reset_seed right below
it in the same file.

New behavior

Same rules, shorter. The contract a shared group has to honour now lives on
seed_group, and _reset_custom_samplers and _validate_custom_sampler point
at it rather than restating it. Inline comments state the constraint rather than
the investigation, and the #1103 reference is kept as a reference.

Nothing outside comments and docstrings changed. I checked that by parsing both
revisions, dropping every docstring, and comparing the resulting ASTs: identical
for both files.

Net effect is -21 lines.

Breaking change

• No

Additional information

Local run against develop at cb6106a:

ruff check .            All checks passed
ruff format --check .   312 files already formatted
pylint rocketpy/ tests/ docs/    exit 0
pytest tests/unit       2036 passed, 12 skipped

One thing I left alone. seed_group's prose is down from 12 lines to 6, but its
body is return self, so it is still longer than the code it sits on. It is an
abstract property that users implement themselves, and the three things it says
(all members return the same generator, return the same object each call, one
group per model) are each a way to get a silently wrong seed. I would rather
keep those than hit a line count. Happy to cut it further if you disagree.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.30%. Comparing base (e0ff281) to head (85d682d).
⚠️ Report is 58 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1154      +/-   ##
===========================================
+ Coverage    82.18%   83.30%   +1.12%     
===========================================
  Files          122      130       +8     
  Lines        16355    17080     +725     
===========================================
+ Hits         13441    14229     +788     
+ Misses        2914     2851      -63     

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