[SERVER] Add section about WT dirty data limit to transactions page

In [HELP-28365], we found that a large uncommitted transaction that exceeds 5% of the allocated WiredTiger cache size will abort the transaction with a WriteConflict error. This problem comes up frequently and it appears users are not aware of this limit. The error message is slightly misleading (we've filed SERVER-60839 to address this), but we should add a section in our documentation to alert users of this limit.

(Feel free to modify the title of this ticket as needed)