docs: clarify ignored terminals with LALR

[Mirochill]

Summary

• Document that LALR skips ignored terminals before parsing.
• Clarify that ignored terminals should not be referenced from LALR rules when their position matters.
• Note the Earley/LALR difference discussed in Ignored terminal used in the grammar is expected with Earley but not with LALR  #1473.

Fixes #1473

Validation

• git diff --check
• Not run locally beyond the static diff check; documentation-only change.