[mlir][Transforms] Dialect conversion: Improve signature conversion API (#81997)

This commit improves the block signature conversion API of the dialect
conversion.

There is the following comment in
`ArgConverter::applySignatureConversion`:
```
// If no arguments are being changed or added, there is nothing to do.
```

However, the implementation actually used to replace a block with a new
block even if the block argument types do not change (i.e., there is
"nothing to do"). This is fixed in this commit. The documentation of the
public `ConversionPatternRewriter` API is updated accordingly.

This commit also removes a check that used to *sometimes* skip a block
signature conversion if the block was already converted. This is not
consistent with the public `ConversionPatternRewriter` API; blocks
should always be converted, regardless of whether they were already
converted or not.

Block signature conversion also used to be silently skipped when the
specified block was detached. Instead of silently skipping, an assertion
is triggered. Attempting to convert a detached block (which is likely an
erased block) is invalid API usage.