feat(modem): add ability to change ESP_MODEM_C_API_STR_MAX from Kconfig

docs/esp_modem/en/api_docs.rst

@@ -43,7 +43,7 @@ These functions are the actual commands to communicate with the modem using AT c
 
 Note that the functions which implement AT commands returning textual values use plain ``char *``
 pointer as the return value. The API expects the output data to point to user allocated space of at least
-``ESP_MODEM_C_API_STR_MAX`` (64 by default) bytes, it also truncates the output data to this size.
+``CONFIG_ESP_MODEM_C_API_STR_MAX`` (128 by default) bytes, it also truncates the output data to this size.
 
 .. doxygenfile:: esp_modem_api_commands.h