parser: clarify that multiline parser does not join separate events (#649)

parser: clarify that multiline parser does not join separate events

Closes: #338
Signed-off-by: Shizuo Fujita fujita@clear-code.com

parser/filter: refer to  under 

The argument of <filter> is a tag pattern, so <filter parser> reads
as a filter for the tag “parser”. Describe it as the <parse> section
under <filter> instead.
Also add a note to filter/parser.md. The existing wording says that you
can reuse the predefined formats there, which does not hold for
multiline.
Signed-off-by: Shizuo Fujita fujita@clear-code.com

Signed-off-by: Shizuo Fujita fujita@clear-code.com