x/authz spec improvement: add pruning of expired grants feature details

[dalmirel]

Summary of Issue

During the specification and code inspection of authz module (as a part of the audit process that includes model-based testing) I have found that specification markdown files are missing important information (in my opinion) about pruning of expired grants' logic in Begin Blocker function.
It was agreed with @AmauryM to open an issue for a suggestion to add additional details, in order to eliminate the need for seeking feature explanations in code implementation.

Since the pruning is mentioned here:

"We are maintaining a queue for authz pruning, whenever a grant created an item will be added to GrantQueue with a key of granter, grantee, expiration and value added as array of msg type urls."

maybe this is the best place(?).

Version

master (commit: 0b810ba)

auditing is performed for tag: v0.46.0-alpha4 (commit: 354faa8)