Clarification for LEAD/CONTACT view retention in SFDC

Why is this change being made?

Add clarification that LEAD/CONTACT views in SFDC are retained for no more than 90 days.

/mention https://gitlab.com/gitlab-com/marketing/marketing-operations/issues/114

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what