Multi-select issue cards documentation needs a review and minor changes

The feature "Multi-select issue cards" was added in GitLab 12.4, and documented.

Issue

• The GitLab version in which the feature was added needs to be added to the feature documentation.
• The documentation was not reviewed by a technical writer, so a review post-merge is also required.
• The instructions mention "left click" but this is not standard on MacOS. Suggestion: change "left-click" to "click".
• Ensure it's clear the feature is available in GitLab Core.