Document createMessageHelper() SDK API for mobile messaging

[jaredlockhart]

The createMessageHelper() method on NimbusInterface is the core SDK API used by the mobile messaging system for targeting evaluation and string substitution, but it's not documented in experimenter-docs.

Missing documentation:

• createMessageHelper(additionalContext?) — factory method returning NimbusMessagingHelperInterface
• evalJexl(expression) — evaluate JEXL targeting expressions
• stringFormat(template) — string substitution with context attributes
• getUuid(template) — generate stable UUIDs for templates
• destroy() / clearCache() — lifecycle management