Improve public api docs

[dkamburov]

Is your feature request related to a problem? Please describe.

Make sure the public api summary and @remarks notes do not contain angular specific items. we should use generic descriptions and not mention selectors or tags or contain code snippets. Transform those into plain text.

For example:
///

/// Returns IgxGridRow object by the specified primary key.
/// @remarks
/// Requires that the primaryKey property is set.

should be
///

/// Returns row object by the specified primary key.
/// @remarks
/// Requires that the primary key is set.

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