docs: Fix a few typos

[timgates42]

There are small typos in:

• docs/extensions.rst
• docs/upgrade.rst
• tinydb/queries.py

Fixes:

• Should read truncate rather than trunacte.
• Should read synchronous rather than syncronous.
• Should read function rather than fuction.
• Should read explicitly rather than explicitely.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[msiemens]

Thanks <3