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.

[JackSpiece]

Verified this against the behavior reported in #496 with temporary DOCX templates. {{ soil.pop }} resolves to the dictionary method, autoescape=True renders that method representation as text rather than selecting the key, and {{ soil['pop'] }} renders 2 as intended. Without autoescaping, the method representation is interpreted as XML and the visible value is lost.

I also ran the exact project flake8 command, git diff --check, and a Sphinx dummy build with warnings treated as errors. All passed. This documents the workaround the owner requested and the placement is appropriate. Looks good.