Clarify which Smdbltrp SRET behaviors are qualified by M-mode

[aswaterman]

See #2870

[aswaterman]

@Timmmm

[github-actions]

Normative Rule Changes Detected

This PR modifies normatively tagged text. Please review the changes below to ensure they are intentional.

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Normative Tag Change Report

riscv-spec Specification

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Tag Changes Report
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Reference file: ref/riscv-spec-norm-tags.json
Current file: build/riscv-spec-norm-tags.json
Modified 1 tag:
  * "norm:mstatus_mdt_clr_mret_sret":
      Reference: "The MRET and SRET instructions, when executed in M-mode, set the MDT bit to 0."
      Current:   "The MDT bit is set to 0."

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Summary: 1 total changes
  Added:    0
  Deleted:  0
  Modified: 1
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---

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How to update reference files (if needed):

make update-ref
git add ref/*.json
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[ved-rivos]

LGTM. Left three suggestions.

[ved-rivos]

Suggested change

	[#norm:sstatus_sdt_clr_mret_sret]#If the new privilege mode is U, VS, or VU, then `sstatus.SDT` is also set to 0.#
	[#norm:sstatus_sdt_clr_mret_sret]#If the Ssdbltrp extension is also implemented, and the new privilege mode is U, VS, or VU, then `sstatus.SDT` is also set to 0.#

[ved-rivos]

Suggested change

	[#norm:mstatus_mdt_clr_mnret]#When the Smdbltrp extension is implemented, the `MNRET` instruction, provided by the Smrnmi extension, sets the `MDT` bit to 0 if the new privilege mode is not M.#

[ved-rivos]

Suggested change

	[#norm:sstatus_sdt_clr_mnret]#If the Ssdbltrp extension is also implemented, and the new privilege mode is U, VS, or VU, then `sstatus.SDT` is also set to 0.#