Add warning not to import untrusted data in Variable Explorer

[jitseniesen]

Issue Report

Issue Description

Importing untrusted data in the variable explorer is unsafe because it uses pickle which executes arbitrary code. Users should be warned against this.

Where is the relevant portion of the documentation located?

Second line of https://github.com/spyder-ide/spyder-docs/blob/master/doc/panes/variableexplorer.rst#toolbar-buttons

What would you suggest it be replaced with?

Add a warning.

Other information

• Is the problem specific to documentation for a particular version of Spyder?

  • [x] N/A
  • [ ] Spyder 5 (master)
  • [ ] Spyder 4 (4.x)
  • [ ] Spyder 3 (3.x, frozen)

• Are you planning on submitting a Github pull request with a suitable change?

  • [ ] Yes
  • [x] No
  • [ ] Maybe so

[CAM-Gerlach]

Thanks for documenting this; I've opened #330 to add it.