Expand Mail documentation with DSL examples

The Mail module documentation previously only showed XML configuration
examples. This update adds examples for Java, Java DSL, Kotlin DSL,
and Groovy DSL to help developers using these different configuration
styles.

Changes include:
- Outbound channel adapter configuration in all DSL flavors
- Header enricher examples for all supported languages
- Inbound adapters (both polling and IDLE) with DSL examples
- JavaMail properties configuration examples
- Search term strategy configuration examples
- Mail filter expression examples across all DSL types
- Transaction synchronization examples for all configurations
- Minor typo fix in SearchTerm documentation
- Minor grammar fixes

This makes the documentation more accessible to developers who prefer
annotation-based or DSL configuration over XML.

Note:
In some cases there is no DSL for java, kotlin, or groovy.
This is because the sample is for a supporting bean of the DSL vs. the DSL itself.

Author: cppwfs