Add note in docs for docker executor for interactive web terminal

What does this MR do?

Add a notice about the docker executor, when using the interactive web terminal. Right now the docker executor does not support the feature of wait for the terminal when the job build script is finished. gitlab-runner#3605 goes into more detail as of why.

Related issues

gitlab-runner#3467 (closed)

gitlab-runner#3605

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards