chore: Silence clippy warning with nightly

[hlinnaka]

The warning:

warning: first doc comment paragraph is too long
  --> pageserver/src/tenant/checks.rs:7:1
   |
7  | / /// Checks whether a layer map is valid (i.e., is a valid result of the current compaction algorithm if no...
8  | | /// The function checks if we can split the LSN range of a delta layer only at the LSNs of the delta layer...
9  | | ///
10 | | /// ```plain
   | |_
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#too_long_first_doc_paragraph
   = note: `#[warn(clippy::too_long_first_doc_paragraph)]` on by default
help: add an empty line
   |
7  ~ /// Checks whether a layer map is valid (i.e., is a valid result of the current compaction algorithm if nothing goes wrong).
8  + ///
   |

Fix by applying the suggestion.

[github-actions]

5065 tests run: 4901 passed, 0 failed, 164 skipped (full report)

---

Flaky tests (10)

Postgres 17

• test_pageserver_compaction_smoke: release-x86-64
• test_subscriber_restart: debug-x86-64
• test_timeline_archive[4]: debug-x86-64

Postgres 16

• test_slots_and_branching: release-arm64
• test_subscriber_restart: release-x86-64

Postgres 15

• test_multi_attach: release-x86-64

Postgres 14

• test_gin_redo: release-arm64
• test_hot_standby_feedback: release-arm64
• test_location_conf_churn[3]: release-x86-64
• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 32.0% (7491 of 23395 functions)
• lines: 50.0% (60475 of 120849 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
b37d204 at 2024-09-25T21:40:31.892Z :recycle:}

[tristan957]

kicked your CI