Clarify creating plugin SVN tags from trunk

[ekamran]

Summary

Clarifies the "Create tags from trunk" guidance in the Plugin Handbook's Using Subversion page.

Changes

• Removes guidance that suggests checking out the repository root locally.
• Recommends working from a local checkout of trunk/.
• Adds a remote svn cp example for creating a release tag from trunk.
• Adds svn sw examples for switching between a tag and trunk when inspection is needed.

This follows up WordPress/Documentation-Issue-Tracker#78.

Note: #17 also touches this file, but it adds a separate troubleshooting section and does not address this tag-creation guidance.