docs: note default branch requirement for automatic PR linking

[BaconMan1168]

Why:

Closes: #45948

The article says a pull request created from a linked branch is automatically linked to the issue. In practice that only happens when the pull request targets the repository's default branch, so the current wording is inaccurate.

What's being changed (if available, include any code snippets, screenshots, or gifs):

In creating-a-branch-for-an-issue.md, the "About branches connected to an issue" paragraph now says:

• automatic linking applies when the pull request targets the default branch
• if it targets a different branch, the pull request is not linked automatically and the branch stays in the "Development" section
• the pull request can be linked manually, with a pointer to the existing "Linking a pull request to an issue" article

One paragraph changed, one file.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
issues/tracking-your-work-with-issues/using-issues/creating-a-branch-for-an-issue.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[subatoi]

Thank you for opening a PR for this, but since this hasn't been approved for a contribution yet, I'm going to close this out. You can read more about contributing to the GitHub docs here