chore: Clarify Exemplar applicability to async instruments

[cijothomas]

The Exemplar section's intro and bullet list currently focus heavily on trace-metric correlation, which can give the impression that Exemplars are only useful with synchronous instruments (where an active span is typically present). This PR makes two small clarifications:

Changes:

• Add a sentence to the intro noting that Exemplars also preserve attributes dropped during aggregation, regardless of instrument type (including asynchronous instruments)
• Qualify the trace/span ID bullet with "For synchronous instruments" to make it clear this aspect is specific to sync instruments, rather than a general requirement

[jack-berg]

Huh.. we don't collect exemplars for asynchronous instruments, but I didn't consider the angle of capturing attributes dropped during aggregation.

[cijothomas]

Huh.. we don't collect exemplars for asynchronous instruments, but I didn't consider the angle of capturing attributes dropped during aggregation.

You mean "OTel Java" implementation don't collect exemplars right?