docs(config): update editor and browser config docs

Description

• Updates glab config documentation to make it clear that env variables override config values.

• Make capitlization consistent

Related Issues

Addresses #7480

How has this been tested?

glab config --help

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap