Use readable names for cluster channel/map options in documentation AND sample configurations

[archan0621]

Fix 62814: Use readable names for cluster channel/map options in documentation AND sample configurations

This patch updates several clustering documentation files to clarify that
both channelSendOptions and mapSendOptions accept human-readable option
names in addition to the existing integer bitmask values.

Tomcat already supports readable names (e.g., "async", "sync,use_ack"),
but the current documentation focuses primarily on numeric flags and
does not explicitly mention that mapSendOptions also supports the same
syntax. This can lead to confusion when configuring BackupManager or
interpreting sample configurations.

Changes included:

• cluster-howto.xml: Updated explanations of send options and added
  references to readable option names. Improved consistency with the
  configuration reference.
• cluster-manager.xml: Expanded the mapSendOptions attribute description
  to document readable names and clarify default behavior.
• cluster-valve.xml: Minor adjustments to align terminology with the
  updated clustering documentation.

No functional code changes are included in this patch. This is a
documentation update only.

Please let me know if further adjustments or additional examples are
desired. I am happy to iterate on the patch.