Removes misleading example as instructions were already provided.

What does this MR do?

Propose a solution to #35750 (closed).

Currently the documentation might suggest that the same format VARIABLE_NAME=VALUE1,VALUE2 could be acceptable for both environmental variables and YAML files. YAML requires a key value format. This affects both SAST and Dependency Scanning.

Those variables are described in a section of the document which already contains a link to how they can be configured:

• https://docs.gitlab.com/ee/user/application_security/dependency_scanning/index.html#available-variables
• https://docs.gitlab.com/ee/user/application_security/sast/#available-variables

The other variables have their possible values/formats within the description column, so this MR changes SAST_EXCLUDED_PATHS and DS_EXCLUDED_PATHS to follow similar alignment.

Note that from a YML perspective, those values will be strings not arrays.