doc: fix exec stdout & stderr default type in child_process

[sartrey]

Checklist

• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

Description of change

add words in child_process docs about invalid Buffer encoding leading to a default buffer output

[sartrey]

#6666

[cjihrig]

I think this should say something like:

If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to

[cjihrig]

Also, please wrap lines at 80 characters.

[sartrey]

@cjihrig I will edit these lines.

[cjihrig]

With a quick check, it seems like these lines are still > 80 characters.

[sartrey]

@cjihrig
Yeah, these lines are still > 80 characters.
However, I found there are some long lines before my change.
I try to find balance between line wrap & easier to read.
Please tell me if I should keep wrap at accurate 80 characters.

Sorry for my poor English, here is my true meaning in Chinese

我在文档的前半部分看到一些行也超过了80字符，
看上去可能是要保持一行意义完整。
所以，我也尝试在一行里取得某种平衡。
改动后的行大约80字符左右，可能有一点超过。

如果我必须精确遵循80字符，请告诉我，我会修改的。

[cjihrig]

Please wrap them at 80 characters or less. There are some other longer lines, but they shouldn't be that way (no need to change the other ones in this PR though).

[sartrey]

@cjihrig Please review this new commit.

[cjihrig]

LGTM

[cjihrig]

Thanks! Landed in 926707f.