docs: clarify dictionary method keys

[janmarsino98]

Summary

• Document bracket notation for dictionary keys that collide with dictionary methods.
• Clarify that autoescaping can prevent invalid XML but does not select the mapping key.

Why

{{ soil.pop }} resolves to the dictionary method rather than the pop context key. {{ soil['pop'] }} renders the intended value.

Fixes #496

Validation

• git diff --check
• Rendered a temporary DOCX with {{ soil['pop'] }} and confirmed that it reopens with the expected value.