Add context for Central encrypted forms feature

In https://docs.getodk.org/central-encryption/, replace "similar to ODK Aggregate" with a description of when this encryption feature may be useful and what security measures are in place without it.

[This forum post](https://forum.getodk.org/t/ssl-issue-with-central-on-my-internal-network/25648/14?u=ln) suggests it's not very clear.

Open the Central encryption documentation page at docs.getodk.org/central-encryption/ and review the sentence about being similar to ODK Aggregate. Read the linked forum post for the reported lack of clarity, then replace that sentence with an explanation of when the feature is useful and what security measures exist without it.

Written by the indexing model from the issue text.