Document Bundler checksum behavior for default gems

[p-linnane]

What was the end-user or developer problem that led to this PR?

Bundler's own checksum is only included when the matching .gem file is cached. Bundler installed as a default gem may not have this file, but this behavior was not documented.

What is your fix for the problem, implemented in this PR?

Document the limitation under lockfile_checksums and regenerate bundle-config(1). This does not change runtime behavior.

@Edouard-chin, you mentioned in #9512 that you would be happy to review this, so here it is 😄

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• Describe the problem / feature
• Write tests for features and bug fixes (N/A, documentation only)
• Write code to solve the problem (N/A, documentation only)
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