Mention cases when @noindent = @indent

[積丹尼 Dan Jacobson]

Add a note to (info "(texinfo) @noindent")
mentioning why

$ echo -e @indent\\n\\nwwww|makeinfo --plaintext

   wwww
$ echo -e @noindent\\n\\nwwww|makeinfo --plaintext

   wwww

give the same output. (Don't tell me. I know why. Mention why in the
document. Thanks.)