Fix incorrect doc for stdio_put_string

[anoshyn]

Summary

The doxygen for stdio_put_string in src/rp2_common/pico_stdio/include/pico/stdio.h reads "This method returns as soon as input is available, but more characters may be returned up to the end of the buffer." That text was copy-pasted from an input-reading helper (e.g. stdio_get_until); stdio_put_string is output-only and never returns early on its own. The wording is actively misleading because the function has an int return value, so a reader could plausibly assume some short-write behavior.

Rewrite the description to match the implementation in stdio.c:92-114:

• writes the buffer to every registered driver in turn (no early return);
• documents the len == -1 → strlen convention;
• documents cr_translation behavior (applies to any appended newline too);
• documents the 0-return edge case when PICO_STDIO_IGNORE_NESTED_STDOUT is set and a concurrent stdout write is in progress.

No behavior change; doxygen only.

Fixes #2818.

Test plan

• git diff — only src/rp2_common/pico_stdio/include/pico/stdio.h is touched; only the doxygen block for stdio_put_string.
• CI builds pass (header still compiles cleanly).
• Doxygen renders without warnings for stdio_put_string.

[anoshyn]

@kilograham @lurch when you have a moment, would you mind taking a look? This is a doxygen-only change to stdio_put_string that fixes the description copy-paste flagged in #2818. Happy to adjust wording if you'd prefer different phrasing. Thanks!

[lurch]

Hi @anoshyn , as requested in other PRs, if you could update all of your pico-sdk PRs to remove the "Co-Authored-By: Claude" lines, that'd be helpful 👍

[anoshyn]

Force-pushed: dropped the Co-Authored-By trailer, and trimmed the prose body so it no longer restates the \param lines - now just a brief plus two short sentences covering what isn't in the params (full-buffer write, may block under back-pressure). New HEAD: c2a8ef1.

[lurch]

I think I'd be tempted to do:

Suggested change

	* Writes \p s in full to every registered stdio driver. The call returns once
	* Writes \p s to every registered stdio driver. The call returns once

as you could give a len shorter than s if you didn't want to write all of it. (But this is optional, given that Graham has already approved this PR.)

[anoshyn]

Good catch — applied in 3dc5e04. "in full" did contradict the \param len line right below it.