[SaaS] User cap for groups settings text should be SaaS-specific

Problem summary

Currently, the text explanation under the group settings for User cap feature is self-managed specific, with mention of "instance" and "administrator", which is not relevant in the context of the SaaS product.

Proposal

The text for this feature should be aligned with the terminology used with the SaaS product.

Suggested text:

After the group reaches the user cap, any user who is added or requests access must be approved by a group Owner. Leave empty for an unlimited user cap. If you change the user cap to unlimited, you must re-enable project sharing and group sharing. Increasing the user cap does not automatically approve pending users.