doc: remove dangling reference to deleted lemma in asymptotics docstring

[kim-em]

This PR rewords the Bounded Range versus IsBigO Asymptotics section docstring in Mathlib/Analysis/Asymptotics/SpecificAsymptotics.lean, which referenced Function.Even.isBigO_atTop_iff_isBigO_atBot, a lemma deliberately removed during review and not present anywhere in Mathlib. The new text describes what the section actually contains, including the main general result Continuous.isBounded_range_iff_isBigO (previously unmentioned).

Follow-up to #40664 (feat: Bounded Range versus IsBigO Asymptotics).

🤖 Prepared with Claude Code

[github-actions]

PR summary 0575e2710b

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 0575e27).

• +0 new declarations
• −0 removed declarations

No declaration differences.

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit 0575e2710b
Reference commit a33a5ccd63

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).