Document the limitation around field aliases and percolator queries.

docs/reference/mapping/types/alias.asciidoc

@@ -68,6 +68,10 @@ There are a few restrictions on the target of an alias:
 Additionally, a field alias can only have one target. This means that it is not possible to use a
 field alias to query over multiple target fields in a single clause.
 
+An alias can be changed to refer to a new target through a mappings update. A known limitation is that
+if any stored percolator queries contain the field alias, they will still refer to its original target.
+More information can be found in the <<percolator, percolator documentation>>.
+
 [[unsupported-apis]]
 ==== Unsupported APIs
 

docs/reference/mapping/types/percolator.asciidoc

@@ -743,3 +743,11 @@ The script inside a `script` query can only access doc values fields. The `perco
 into an in-memory index. This in-memory index doesn't support stored fields and because of that the `_source` field and
 other stored fields are not stored. This is the reason why in the `script` query the `_source` and other stored fields
 aren't available.
+
+[float]
+===== Field aliases
+
+Percolator queries that contain <<alias, field aliases>> may not always behave as expected. In particular, if a
+percolator query is registered that contains a field alias, and then that alias is updated in the mappings to refer
+to a different field, the stored query will still refer to the original target field. To pick up the change to
+the field alias, the percolator query must be explicitly reindexed.