doc: replace newlines in deprecation with space

As it is, each line in the deprecation heading which are wrapped at 80 characters in the *.md files, are shown in different lines. For example

> Stability: 0 - Deprecated: Use
> `Buffer.from(arrayBuffer[, byteOffset [, length]])`
> instead.

is shown in three different lines. This patch replaces the newlines with space characters, so that the output will be in single line.

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Before this patch:

https://nodejs.org/api/buffer.html#buffer_new_buffer_arraybuffer_byteoffset_length

After this patch:

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

tools

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cc @nodejs/documentation