Docs: add dav_subspace to the PW GPU solver list

[Critsium-xy]

Reminder

• I have read AGENTS.md and docs/developers_guide/agent_governance.md.
• I have linked an issue or explained why this PR does not need one.
• I have added adequate unit tests and/or case tests, or explained why not.
• I have listed the exact verification commands run and their results.
• I have described user-visible behavior changes, including INPUT parameter changes.
• I have explained core-module impact for ESolver, HSolver, ElecState, Hamilt, Operator, Psi, or other source/ changes.
• I have requested any needed governance exception below.

Linked Issue

Fix #7775

Unit Tests and/or Case Tests for my changes

• Commands run:
  • test -d tests/11_PW_GPU/scf_dav_sub
  • rg -n dav_subspace docs/advanced/acceleration/cuda.md tests/11_PW_GPU/scf_dav_sub/INPUT
  • git diff --check
  • python3 tools/03_code_analysis/agent_governance_check.py --staged
  • python3 tools/03_code_analysis/agent_governance_check.py --base upstream/develop --head HEAD --format text
• Result summary: all commands passed; both governance checks reported no findings.
• Checks not run, with reason: no ABACUS build or GPU runtime test was run because this PR changes only hand-written documentation and the existing GPU integration case already records ks_solver dav_subspace.

What changed?

• Document dav_subspace as a supported PW GPU eigensolver.
• Correct the grammar and punctuation of the same solver-list sentence.

Governance Notes

• INPUT/docs changes: documentation-only clarification; no INPUT behavior or generated parameter documentation changes.
• Core module impact: none.
• Exceptions requested: none.

[mohanchen]

LGTM