[SPARK-40028][SQL] Add binary examples for string expressions

[beliefer]

What changes were proposed in this pull request?

Currently, Spark have many string expressions support binary type, but missing examples of binary.

This PR will add examples of binary for some string expressions show below. Elt Contains StartsWith EndsWith StringLPad StringRPad Substring Left Length BitLength OctetLength Base64

Why are the changes needed?

Add binary examples for string expressions

Does this PR introduce any user-facing change?

'No'. Just update the document for some string expressions.

How was this patch tested?

ExpressionInfoSuite

[beliefer]

GA failure unrelated with this PR. ping @MaxGekk @gengliangwang @dongjoon-hyun @HyukjinKwon cc @cloud-fan

[HyukjinKwon]

Merged to master.

[beliefer]

@HyukjinKwon @gengliangwang @MaxGekk Thank you!

[gengliangwang]

Created a followup for this one https://github.com/apache/spark/pull/37462