[PATCH] manual: Update memchr description [BZ #19406]

[Andreas Schwab]

On Jun 15 2026, Wilco Dijkstra wrote:

> +This function behaves as if each byte is read sequentially and stops as
> +soon as the first match is found.  @var{size} may be larger than the object

Please avoid starting a sentence with a non-regular word.

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Andreas Schwab, SUSE Labs, schwab@suse.de
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