Document dynamic import support [skip ci]

[guybedford]

Q	A
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Deprecations?
Spec Compliancy?
Tests Added/Pass?
Fixed Tickets
License	MIT
Doc PR	Yes
Dependency Changes

This was previously undocumented while still experimental, but at this point I think it would be great to ensure the information is properly provided now.

[mention-bot]

@guybedford, thanks for your PR! By analyzing the history of the files in this pull request, we identified @xtuc, @existentialism and @hzoo to be potential reviewers.

[codecov]

Codecov Report

Merging #5457 into 7.0 will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             7.0    #5457      +/-   ##
=========================================
- Coverage   85.4%   85.36%   -0.05%     
=========================================
  Files        203      203              
  Lines       9525     9525              
  Branches    2703     2703              
=========================================
- Hits        8135     8131       -4     
- Misses       903      907       +4     
  Partials     487      487

Impacted Files	Coverage Δ
packages/babel-helper-call-delegate/src/index.js	64% <0%> (-4%)	❌
packages/babel-traverse/src/path/modification.js	73.78% <0%> (-0.98%)	❌
packages/babel-traverse/src/path/context.js	84.48% <0%> (-0.87%)	❌
...bel-plugin-transform-es2015-classes/src/vanilla.js	89.74% <0%> (-0.86%)	❌
packages/babel-traverse/src/visitors.js	86.66% <0%> (+0.95%)	✅

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