Improve docs on trunk runtimes

[rindeal]

The runtimes section contains contradictory statements.

Below is a rundown generated by LLM:

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Here are a couple of points that seem contradictory:

1. Hermetic Installation vs. System Re-use:

   • The first statement says, "Trunk manages the hermetic installation of all required runtimes," which implies that Trunk will always use its own isolated runtime installations.
   • However, it also mentions that you can tell Trunk to re-use an already-installed runtime on the system, which contradicts the idea of hermetic (isolated) installations.

2. Default Behavior vs. Version Constraints:

   • It states, "By default, Trunk will use its own hermetic runtime installations," suggesting that Trunk will not use system-installed runtimes unless specified otherwise.
   • Yet, it also says, "To specify that Trunk is allowed to use the version of a runtime available in your PATH, you can specify a range-based version constraint," which implies that Trunk can use system-installed runtimes if they meet the specified constraints.

3. Example Configuration:

   • The example configuration shows that Trunk will use a system-installed version of Ruby if it matches >=2.7.1 and Python if it matches 3.9.1 exactly.
   • This seems to contradict the earlier statement that Trunk will use its own hermetic installations by default.

These points create some confusion about whether Trunk prioritizes its own hermetic installations or if it can use system-installed runtimes under certain conditions. It might be helpful to clarify the default behavior and the conditions under which system-installed runtimes are used.