kamailio-kemi-framework: added doc files for a few htable functions

kamailio-kemi-framework/docs/modules/htable/htable.header.md

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+Functions exported by `htable` module.

kamailio-kemi-framework/docs/modules/htable/htable.sht_dec.md

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+Do atomic decrement to the item value. It returns the new value or `-255`
+if the hash table does not exist, or the item does not exist or the item value
+is not integer.

kamailio-kemi-framework/docs/modules/htable/htable.sht_inc.md

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+Do atomic increment to the item value. It returns the new value or `-255`
+if the hash table does not exist, or the item does not exist or the item value
+is not integer.