Correct fold() reference example for list vs map seed with addAll

[spmallette]

The Fold Step section of the reference documentation annotated the example g.inject(["a":1],["b":2]).fold([], addAll) as "a mechanism for merging Map instances" where a later key occurrence replaces an earlier one. That description does not match what the line does. With a list seed ([]), addAll collects the incoming maps into a list, producing [[a:1],[b:2]] with no merging and no key replacement. The described last-writer-wins merge only happens with a map seed.

This change:

• Keeps the existing list-seed example, which is correct and useful.
• Rewrites its callout to describe the actual list-seed behavior (collect the maps into a list, [[a:1],[b:2]], no merge).
• Adds a companion map-seed example, g.inject(["a":1],["b":2]).fold([:], addAll), with a callout describing the real putAll merge ([a:1,b:2], last occurrence wins).
• Makes the seed-type dependence of addAll explicit: a list seed collects elements into a list, while a map seed merges the maps.

Both examples are executed at doc-build time. The rendered outputs confirm the documented results: the list seed yields [[a:1],[b:2]] and the map seed yields [a:1,b:2]. Callouts 1-6 and unrelated content are unchanged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.41%. Comparing base (a28cd1f) to head (9c200eb).
⚠️ Report is 347 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3554      +/-   ##
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+ Coverage     76.35%   76.41%   +0.05%     
- Complexity    13424    14280     +856     
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  Files          1012     1036      +24     
  Lines         60341    64561    +4220     
  Branches       7075     7656     +581     
============================================
+ Hits          46076    49336    +3260     
- Misses        11548    12132     +584     
- Partials       2717     3093     +376     

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