API Fuzzing: Add new variable OVERRIDES_CMD_VERBOSE to documentation

What does this MR do?

This MR documents the support to a new environmental variable FUZZAPI_OVERRIDES_CMD_VERBOSE. This variable has been created to allow users to show or hide the output of external commands. The external command is provided by FUZZAPI_OVERRIDES_CMD (already documented).

In the related MR by default output of the command is hidden, unless an error is detected. However, if FUZZAPI_OVERRIDES_CMD_VERBOSE is set to any value (but empty), then the output of the comment is shown as part of the job output.

Additionally, this MR adds an example of how to produce a log file related to the override command.

Related issues

#334578 (closed)

Depends on MR

https://gitlab.com/gitlab-org/security-products/analyzers/api-fuzzing-src/-/merge_requests/537

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