Documentation is missing quotes for `partial_sync`

When the parameter of partial_sync is not quoted, e.g. partial_sync = ignore instead of partial_sync = "ignore", then the following error rises:

 option "partial_sync": Expecting value: line 1 column 1 (char 0)

However, the documentation indicating that the parameter should be used without quotes, so either the program or the documentation should be adopted.

Relevant sections are:

• http://vdirsyncer.pimutils.org/en/stable/config.html#pair-section (quote the possible choices for the parameter in the bullet list like in the other parameters)
• http://vdirsyncer.pimutils.org/en/stable/partial-sync.html#step-3-the-partial-sync-parameter

Read the pair-section and Step 3 sections linked in the issue, then compare their partial_sync examples with the reported parser error. Update the relevant documentation so the parameter choices and quoting are unambiguous, and verify both sections show consistent syntax.