parser/filter: refer to <parse> under <filter> accurately

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>

Author: Watson1978

filter/parser.md

@@ -20,6 +20,10 @@ It is included in the Fluentd's core.
 
 `filter_parser` uses built-in parser plugins and your own customized parser plugin, so you can reuse the predefined formats like `apache2`, `json`, etc. See [Parser Plugin Overview](../parser/) for more details
 
+{% hint style='warning' %}
+`filter_parser` receives events which are already split, so `multiline` cannot join them into one record. See [`multiline`](../parser/multiline.md) for the details.
+{% endhint %}
+
 With this example, if you receive this event:
 
 ```text

parser/multiline.md

@@ -8,7 +8,7 @@ The `multiline` parser parses log with `formatN` and `format_firstline` paramete
 This parser plugin itself does not join multiple lines. The logic to buffer lines and detect the start of each record is implemented in `in_tail`.
 
 * `<parse>` in `in_tail`: works as described in this article.
-* `<filter parser>` and other plugins which receive already-split events: cannot join separate events into one record. It can only parse a value which already contains newline characters.
+* `<parse>` section under `<filter>` \([`filter_parser`](../filter/parser.md)\) and other plugins which receive already-split events: cannot join separate events into one record. It can only parse a value which already contains newline characters.
 
 To concatenate multiple events into one record after the input stage, use [`fluent-plugin-concat`](https://github.com/fluent-plugins-nursery/fluent-plugin-concat).
 {% endhint %}