[5.5] Document missing migrations features

[paulofreitas]

• Expands the Table section to document all available table options
• Fixes a tiny spelling typo
• Changes the $table->nullableTimestamps() column modifier description to state it's just an alias (from Difference between nullableTimestamps() and timestamps() ? ideas#874)
• Updates the Modifying Columns note to list just the changeable columns
• Expands the Renaming Columns section to document the available command aliases
• Updates the Available Index Types section for changes introduced in [5.5] Geospatial improvements framework#21919
• Updates the Dropping Indexes section to include the new $table->dropSpatialIndex() method introduced in [5.5] Geospatial improvements framework#21919
• Minor table formatting adjustments

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