Update upgrade.md

[matthewthomaskelly]

IMPORTANT: All Pull Requests should be connected to an issue, if you don't
have an issue, please start by creating an issue and link it to the PR.

Please provide enough information so that others can review your pull request:

• What existing problem does this PR solve?
  Adds clarity to documentation pertaining to upgrades and when it is necessary to migrate database
• What new feature is being introduced with this PR?
  N/A
• Overview of changes to existing functions if required.
  N/A

Checks

• [ N/A ] All tests succeed.
• [ N/A ] Unit tests added.
• [ N/A ] e2e tests added.
• [ Yes ] Documentation updated.

Closing issues
N/A

[matthewthomaskelly]

Should the folders be /opt/timesketch/shared and /opt/timesketch/docker as discussed?

[matthewthomaskelly]

Should the folders be /opt/timesketch/shared and /opt/timesketch/docker as discussed?

Sorry, wasn't looking at most up to date change