docs: minor migration guide updates

[matthewmayer]

• standardize title case for headings
• clearer headings
• grammar simplification
• present tense
• add link to enforce-unique
• enforce-unique does now include exclude

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.95%. Comparing base (b754dc6) to head (c428144). Report is 1 commits behind head on next.

:exclamation: Current head c428144 differs from pull request most recent head ecb2ca1. Consider uploading reports for the commit ecb2ca1 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #2811 +/- ## ======================================= Coverage 99.95% 99.95% ======================================= Files 2973 2973 Lines 212613 212613 Branches 943 946 +3 ======================================= + Hits 212510 212525 +15 + Misses 103 88 -15 ``` [see 2 files with indirect coverage changes](https://app.codecov.io/gh/faker-js/faker/pull/2811/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=faker-js)

[matthewmayer]

i have updated the info about enforce-unique taking into account everyone's comments.

[ST-DDT]

@RD-CL Thanks for bringing this to our attention. Please contact us at ert@fakerjs.dev with details so that we can look into that. GitHub issues is the wrong place to bring this up, so please refrain from further comments here.