fix: correct typos and grammar in migrating_to_ocis docs (#1536)

[DeepDiver1975]

• fix: correct doubled word in "Migration With LDAP Users" docs

The intro of the "Migration With LDAP Users" section had a doubled word ("connected to to a different backend") and a slightly awkward "either connecting ... or by migrating" construction. Reword for clarity and parallel structure.

Reported in owncloud/migrate_to_ocis#30.

• fix: correct typos and grammar in migrating_to_ocis docs

Fix several language issues found while reviewing the migration guide:

• "oCis" -> "oCIS" (consistent product capitalization)
• "how to setup" -> "how to set up" (verb form)
• "Files of enabled users from in each user's home" -> "Files in each enabled user's home" (broken "from in", redundant phrasing)
• "the users personal Space" -> "the user's personal Space" (possessive)
• "IDM - where" -> "IDM — where" (em dash, consistent with rest of doc)
• "migrating users and groups are divided" -> "the migration of users and groups is divided" (subject/verb clarity)
• add comma before non-restrictive "which" in the ownCloud schema note

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[mmattel]

Needs porting to master.

[phil-davis]

Needs porting to master.

This is a backport of #1536 that was merged to master.

[mmattel]

It would be easier to identify if we would use the convention I implemented for the PR header on backports. That was made intentionally exacly because of this...