Docs: Added migration description for custom analysis file path

Closes #15597
Closes #15556

docs/reference/analysis/charfilters/mapping-charfilter.asciidoc

@@ -4,6 +4,16 @@
 A char filter of type `mapping` replacing characters of an analyzed text
 with given mapping.
 
+[horizontal]
+`mappings`::
+
+    A list of mappings to use.
+
+`mappings_path`::
+
+    A path, relative to the `config` directory, to a mappings file
+    configuration.
+
 Here is a sample configuration:
 
 [source,js]
@@ -14,7 +24,10 @@ Here is a sample configuration:
             "char_filter" : {
                 "my_mapping" : {
                     "type" : "mapping",
-                    "mappings" : ["ph=>f", "qu=>k"]
+                    "mappings" : [
+                      "ph => f",
+                      "qu => k"
+                    ]
                 }
             },
             "analyzer" : {
@@ -27,12 +40,3 @@ Here is a sample configuration:
     }
 }
 --------------------------------------------------
-
-Otherwise the setting `mappings_path` can specify a file where you can
-put the list of char mapping :
-
-[source,js]
---------------------------------------------------
-ph => f
-qu => k
---------------------------------------------------

docs/reference/migration/migrate_2_0/settings.asciidoc

@@ -191,6 +191,12 @@ to check for a custom config location.  The value of the `CONF_DIR`
 variable can be set in the environment config file which is located either in
 `/etc/default/elasticsearch` or `/etc/sysconfig/elasticsearch`.
 
+==== Custom analysis file paths
+
+It is no longer possible to set custom file path outside `CONF_DIR` for `*_path` settings
+in <<analysis-charfilters,char>> or <<analysis-tokenfilters,token>> filters.
+You must specify either relative path to `CONF_DIR` location or absolute path inside `CONF_DIR` location.
+
 ==== `ES_CLASSPATH removed`
 
 The `ES_CLASSPATH` environment variable is no longer used to set the class