Doc: Updated GraphQL settings that only work through env var

[marcotc]

What does this PR do?

Related issue comment: https://github.com/DataDog/dd-trace-rb/issues/4170#issuecomment-3074067411

This PR updates the documentation on how to configure the GraphQL tracing mechanism, since it cannot be changed once first configuration today (the patching is irreversible).

Because auto-instrumentation always patches GraphQL if available at application startup, trying to change the mechanism in the Datadog.configure block does not work.

This PR leaves only the environment variable configuration options publicly documented, since they are the only ones that always work reliably.

The ultimate fix for this patching issues is unfortunately a breaking change, thus needs to wait until the next major release.

Change log entry No.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 97.55%. Comparing base (e171484) to head (6b274a8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4842      +/-   ##
==========================================
- Coverage   97.55%   97.55%   -0.01%     
==========================================
  Files        1490     1490              
  Lines       89156    89155       -1     
  Branches     4713     4713              
==========================================
- Hits        86978    86973       -5     
- Misses       2178     2182       +4     

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[ivoanjo]

Hey, should we go ahead and merge this one?