Documentation incorrectly referenced an install_before_mode = "hard" setting that doesn't exist in the codebase. The actual implementation only has install_before with a single behavior: filtering fuzzy version matches while allowing explicitly pinned versions through.

Changes:

• Removed references to non-existent install_before_mode setting from:
  • CLI help text in src/cli/upgrade.rs
  • User documentation in docs/cli/upgrade.md
  • Man page in man/man1/mise.1
  • Usage spec in mise.usage.kdl
• Simplified language from "In 'soft' mode (default)..." to "This only affects..." since there's only one mode

Before:

In "soft" mode (default), this only affects fuzzy version matches like "20" or "latest".
Explicitly pinned versions like "22.5.0" are not filtered.
Use `install_before_mode = "hard"` setting to filter all versions strictly.

After:

This only affects fuzzy version matches like "20" or "latest".
Explicitly pinned versions like "22.5.0" are not filtered.

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