Document with context behavior for import directive

[bengmur]

Summary

Proposed change:

• Document with context behavior on an import directive.

For example:

in a.nunj:

{% set processed_var = from_context %}

in b.nunj:

{% set from_context = "hello" %}
{% from "./a.nunj" import processed_var with context %}
{{ processed_var }} {# will output "hello" #}

in c.nunj:

{% set from_context = "hello" %}
{% from "./a.nunj" import processed_var %}
{{ processed_var }} {# will not output anything #}

Closes #522.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.

[bengmur]

Appears to relate to #522, #539.

[fofr]

I just found this documentation useful, any reason it's not been merged?

[codecov-commenter]

Codecov Report

Merging #1209 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1209   +/-   ##
=======================================
  Coverage   89.60%   89.60%           
=======================================
  Files          22       22           
  Lines        3041     3041           
=======================================
  Hits         2725     2725           
  Misses        316      316           

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