docs: few tweaks to the migration guide

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[X] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #3615

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

Added titles
Added some extra info
Reworded a few things

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[timdeschryver]

Introduced in #3648

[timdeschryver]

Updated the description because we didn't implement the any type generic