x/tools/gopls: document Rename package feature

[adonovan]

See:

• x/tools/gopls: rename package decl fails in CLI #69582

[gabyhelp]

Related Issues and Documentation

• x/tools/gopls: allow renaming arbitrary package paths #57171
• x/tools/gopls: improvements to package renaming #56184 (closed)
• x/tools/gopls: rename package decl fails in CLI (and is undocumented) #69582
• x/tools/gopls: support renaming a package #41567 (closed)
• x/tools/gopls: implement references on package names #52968 (closed)
• x/tools/gopls: support module-local renaming #32877 (closed)
• x/tools/cmd/gorename: tag and delete it #69360 (closed)
• x/tools/gopls: changing package names returns an error #32149 (closed)

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