Clarify behavior of `strict` for `String`-to-number conversions

In table form:

	`whitespace: false`	`whitespace: true`
`strict: false`	`1.2` `1.2` `1.2@@`	`1.2` `1.2` `1.2@@` `1.2` `1.2` `1.2@@`
`strict: true`	`1.2`	`1.2` `1.2` `1.2` `1.2`

The current wording suggests that the two arguments are mutually exclusive and cannot be both set to true, but the lower-right quadrant suggests otherwise. (Note that both whitespace and strict are also true by default.) This PR reflects the current status.

crystal-lang / crystal

Clarify behavior of `strict` for `String`-to-number conversions #15199