Document that QueryConditionInterface::condition() should not be used to add NULL conditions.

Problem/Motivation

• In #813540: Comparisons involving NULL must never return true, the use of QueryConditionInterface::condition() to add an IS NULL condition is removed in Drupal 8 and deprecated in Drupal 7.
• The API documentation for this method does not indicate anything about the use of NULL.

Proposed resolution

• Add an additional paragraph above the @param documentation of QueryConditionInterface::condition() with the following text:
  

  Do not use this method to test for NULL values. Instead, use QueryConditionInterface::isNull() or QueryConditionInterface::isNotNull().

• Add @see lines at the bottom of the docblock to QueryConditionInterface::isNull() and QueryConditionInterface::isNotNull().

Refer to the doxygen standards for details on the formatting.

Remaining tasks

• Create a patch with the above documentation changes for Drupal 8.

User interface changes

• None.

API changes

• None.