Handle the projects cache better

[raxod502]

Handle part of #1936 by allowing for M-x projectile-invalidate-cache to still clear the global cache even if the user invokes it from outside of any project, and no longer force the user to select a project to invalidate the cache for in that case. Also, document how this command may be used for troubleshooting.

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• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the docs (when adding new project types, configuration options, commands, etc)

[raxod502]

I haven't added tests since this is a fairly small logic change and there aren't existing tests for projectile-invalidate-cache, so I'm not sure offhand what if anything would need to be mocked, but if it is desired, I can look into it and figure it out.

[raxod502]

The snapshot tests appear to also be failing on master, so let me know if I should look at those as well.

[bbatsov]

Thanks!

Seems the tests are failing because of some upstream deprecations - I'll handle those myself.