docs(argcheck): fix typos and wording in doc strings (#5369)

Problem
- `deepmd/utils/argcheck.py` contains multiple doc string typos and
awkward phrases.
- Several repeated descriptions use inconsistent grammar, which makes
the generated documentation harder to read.
- Some DPA-related sections use internal shorthand such as `repinit`,
`repformer`, `repflow`, `g1`, `g2`, `h2`, and `tebd` without enough
explanation for first-time readers.

Change
- Fix spelling and grammar issues in descriptor and fitting doc strings.
- Normalize repeated wording for trainable flags, neighbor-selection
descriptions, and embedding formulas.
- Clarify DPA2/DPA3 terminology while keeping the original internal
names for code-to-doc traceability.
- Keep the change documentation-only with no behavior changes.

Notes
- Verified the file still compiles with `python3 -m compileall
deepmd/utils/argcheck.py`.

Authored by OpenClaw (model: custom-chat-jinzhezeng-group/gpt-5.4)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Improved in-code parameter and argument descriptions for grammar,
clarity, and consistent terminology. Reworded multi-line descriptions,
standardized notation (e.g., identifiers and notation styling), and
clarified expectations for model/descriptor configuration options. These
changes are documentation-only; no behavioral, API, default, or
interface changes were made.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: njzjz-bot