rustdoc: don't strip <p> from stability notes

[lolbinarycat]

I believe the reason we were doing this before was that write_html_fmt adds a \n after each

element.

I have worked around this by making white-space:pre-wrap apply only to the contents of each paragraph, so the inserted whitespace between paragraphs is not included.

Perhaps a better solution would be getting rid of pre-wrap entirely, and instead inserting
events in place of newlines, but that would be a significantly larger change.

fixes #149732

r? @GuillaumeGomez

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @lolbinarycat

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- html::markdown::tests::test_markdown_html_escape stdout ----

thread 'html::markdown::tests::test_markdown_html_escape' (57774) panicked at src/librustdoc/html/markdown/tests.rs:475:9:
assertion `left == right` failed: original: `Struct<'a, T>`
  left: "<p><code>Struct&lt;'a, T&gt;</code></p>\n"
 right: "<code>Struct&lt;'a, T&gt;</code>"
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed::<alloc::string::String, &str>