[docs] Updates to Text LineClamp

[DaveL17]

This is a simple revision that adds to the lineClamp attribute of the <Text> element. The attribute can be set to a value of zero which will display all text. This is a useful option for users that may want to toggle the value.

For Example:

<script>
let lineClamp = 1;
</script>

<Text lineClamp={lineClamp} on:click={() => lineClamp = lineClamp === 1 ? 0 : 1}>
  My text goes here...
</Text>

It makes two simple changes:

• adds a short description to the elements' documentation, and
• modifies the demo range to allow for the value of 0.

Associated issue: #510

[BeeMargarida]

Hey, thank you for this PR!

[DaveL17]

My pleasure. Thanks for SvelteUI!