docs-bug(MatCellHarness): getColumnName returns the name, but altered

### Documentation Feedback

The description of `MatCellHarness.getColumnName()` states that it

> Gets the name of the column that the cell belongs to.

What it DOESN'T say is that if the column name has any symbols in it like dots or backslashes or anything that the [_setNameInput method in CdkColumnDef](https://github.com/angular/components/blob/623b0c94e90f83ba1a91d93137ebad6622e12bdd/src/cdk/table/cell.ts#L139) would consider not CSS friendly, the harness will instead return that column's name with all of the offending characters replaced with dashes

**Example:**

Let's say that I've defined row template as [per point 3 in MatTable's Getting Started](https://material.angular.io/components/table/overview#3-define-the-row-templates) section, and one of the columns is called _"bar.tar"_

`columnsToDisplay = ['foo', 'bar.tar'];`

Later on I'm writing tests and using test harnesses to get a list of that table's columns

```
// cells is of type MatCellHarness[]
const columnNames = await parallel(() => cells.map(cell => cell.getColumnName()));
// columnNames is ['foo', 'bar-tar']
```

As I understand, it's because the cell harness uses the [cell's own CSS class](https://github.com/angular/components/blob/31a754c90405ac3a21858cdd038d56aa7375fabc/src/material/table/testing/cell-harness.ts#L23) to deduct its column's name, which is fair enough, but I do strongly think that this behaviour should be documented

### Affected documentation page

https://material.angular.io/components/table/api#MatCellHarness

Review the MatCellHarness.getColumnName() entry on the affected Material table API page, using the linked CdkColumnDef._setNameInput method and cell harness implementation for context. Update the documentation to explain that CSS-unfriendly column-name characters, such as dots and backslashes, are replaced with dashes, and verify the example behavior is accurately reflected.

Written by the indexing model from the issue text.