Update a documentation for CI_MERGE_REQUEST_DESCRIPTION truncation
This is a follow-up issue to #432846 (closed) to update documentation according to the change introduced in !139605 (merged)
Problem to solve
We need to revise the documentation regarding the predefined variable CI_MERGE_REQUEST_DESCRIPTION. This change will involve truncating it if it exceeds a certain length and introducing a new boolean predefined variable called CI_MERGE_REQUEST_DESCRIPTION_IS_TRUNCATED