Done/WIP for 10.1.0.

https://github.com/glpi-project/glpi/pull/11025

The search option exists. If you open the search option dropdown and search "Description" it shows up as Followups > Description.

There is a plugin for this, but the information isn't the most readable since the historical logs were not meant to be searched, only viewed per asset.
https://github.com/cconard96/glpi-historical-plugin

It isn't quite to the point were I want it in the plugin directory, but it should work for what you need. It will add a "Historical" menu item under Administration that will give you a central location to search all of the historical log entries. You should know that you can only enter text to search the old and new values, and the "Search option" column which is the field being changed can only accept the numeric search ID even though it can display the label like "Name" or "Description".

The new PR is https://github.com/glpi-project/glpi/pull/9692

Already planned/in progress :)
See: https://github.com/glpi-project/glpi/pull/9273

The current PR is only closed because the desired spec was different than what I initially made. It is currently being redone and a new PR will be open when ready.

For the global search, the user can add new columns (even from plugins). It is based on their search display preferences (wrench icon on regular search results page). For lists like the Knowledgebase browse tab, the columns are indeed hard coded currently.

This is possible since GLPI 9.5 using the new Marketplace feature.

The documentation is currently being rewritten in English and updated for the latest GLPI version.

https://glpi-user-documentation.readthedocs.io/fr/latest/

The project itself can be found here if you want to contribute:

https://github.com/glpi-project/doc

The goal is to have the documentation in multiple languages through Transifex like GLPI currently is.

There was a pull request for this but it wasn't received well, so I have a work in progress plugin that adds this. It isn't polished yet but works well enough for most uses.
https://github.com/cconard96/glpi-historical-plugin
If I get time, there may be a first release this weekend.

I'm not sure I follow. You can already search assets and Tickets by Country, Town (city), State, Post/Zip code, street address, etc based on the location assigned to them (They are fields within Locations). Each location can be as general or fine-grained as you need it to be down to specific GPS coordinates. For example you can have a location hierarchy like:
United States > New York > New York City > HQ > Admin Building > Office 100
Or more simply:
HQ > Admin Building > Office 100