PR Comments Include commits to existing PRs

### Problem Statement

Currently, when a user creates a new PR that modifies code related to an existing Sentry issue, the GitHub integration doesn't consistently create comments on the PR referencing the existing issue. This behavior is particularly important for teams who want to track down changes that triggered a regression.

Specific issues identified:
- The GitHub integration only comments on PRs when the initial commit of the PR modifies the faulty code
- Additional commits to an existing PR don't trigger new comments, even if they modify relevant code
- Users expect comments to appear on PRs whenever they modify code related to an existing issue, regardless of when the modification occurs in the PR lifecycle
- this isn't clearly communicate in any docs

https://sentry.zendesk.com/hc/en-us/articles/23803547516315-Why-am-I-not-getting-comments-on-pull-requests-PR

### Solution Brainstorm

Potential approaches to address this issue:

1. **Expand PR comment triggers**:
- Process all commits in a PR, not just the initial one

2. **Improve documentation**:
- Create clearer guidance on how the PR comment feature works
- Document edge cases and limitations to set proper expectations

### Product Area

Unknown

Start by tracing the GitHub integration's pull-request comment handling and review the linked Zendesk article. Reproduce the behavior for an initial commit and for later commits that modify code related to an existing Sentry issue. Done means relevant comments are handled consistently across the PR lifecycle and the documented limitations are clear.

Written by the indexing model from the issue text.