Improve docs for query POST requests (#5428)

Signed-off-by: Bob Shannon <bobs@dropbox.com>

docs/querying/api.md

@@ -88,7 +88,7 @@ The current server time is used if the `time` parameter is omitted.
 
 You can URL-encode these parameters directly in the request body by using the `POST` method and
 `Content-Type: application/x-www-form-urlencoded` header. This is useful when specifying a large
-or dynamic number of series selectors that may breach server-side URL character limits.
+query that may breach server-side URL character limits.
 
 The `data` section of the query result has the following format:
 
@@ -154,7 +154,7 @@ URL query parameters:
 
 You can URL-encode these parameters directly in the request body by using the `POST` method and
 `Content-Type: application/x-www-form-urlencoded` header. This is useful when specifying a large
-or dynamic number of series selectors that may breach server-side URL character limits.
+query that may breach server-side URL character limits.
 
 The `data` section of the query result has the following format:
 
@@ -767,4 +767,4 @@ This takes no parameters or body.
 $ curl -XPOST http://localhost:9090/api/v1/admin/tsdb/clean_tombstones
 ```
 
-*New in v2.1 and supports PUT from v2.9*
+*New in v2.1 and supports PUT from v2.9*