update length() description

Per this JIRA, https://segment.atlassian.net/browse/PROT-4223, the length() method isn't meant to be used with JSON and doing so can result in Destination Filters not working. This update clarifies that.

Author: spencerattick

src/api/public-api/fql.md

@@ -130,7 +130,7 @@ You can use parentheses to group subexpressions for more complex "and / or" logi
 | Function                            | Return Type | Result                                                                                                                                                     |
 | ----------------------------------- | ----------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `contains( s string, sub string )`  | `bool`      | Returns `true` if string `s` contains string `sub`.                                                                                                        |
-| `length( list or string )`          | `number`    | Returns the number of elements in a list or number of bytes (not necessarily characters) in a string. For example, `a`  is 1 byte and`ア` is 3 bytes long. |
+| `length( list or string )`          | `number`    | Returns the number of elements in a list or number of bytes (not necessarily characters) in a string. For example, `a`  is 1 byte and`ア` is 3 bytes long. Please note that this function is not meant to be used with JSON as the argument and that doing so may result in unexpected consequences. |
 | `lowercase( s string )`             | `string`    | Returns `s` with all uppercase characters replaced with their lowercase equivalent.                                                                        |
 | `uppercase( s string )`             | `string`    | Returns `s` with all lowercase characters replaced with their uppercase equivalent.                                                                        |
 | `snakecase( s string )`             | `string`    | Returns `s` with all space characters replaced by underscores. For example, `kebabcase("test string")` returns `test_string`.                                                                        |