Java high-level REST client docs

[olcbean]

Few of the API supported by the high-level REST client, do not have docs :

• ping (Docs: HighLevelRestClient#ping #29070)
• exists (Docs: HighLevelRestClient#exists #29073)
• multi get (Docs: HighLevelRestClient#multiGet #29095)
• multi search (Docs: HighLevelRestClient#multiSearch #29144)

Relates to #27205