Clean up String file

[gewarren]

Nothing substantive, just cleanup.

• Replace %28 and %29 with parentheses in xref links
• Remove "ing" from headings
• Remove extra whitespace in bullets
• Add hard newlines where needed
• Remove "the" from .NET Framework

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[opbld33]

Docs Build status updates of commit 2f4803e:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/String.xml	✅Succeeded	View

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[BillWagner]

😓 This is quite a bit of work. I reviewed all the changes and you can when ready.

[gewarren]

Thank you Bill, sorry for the huge PR!