kamailio-kemi-framework: docs for some kx functions

kamailio-kemi-framework/docs/modules/kx/kx.get_body.md

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+Return the body of the SIP message (the value of $rb).
+

kamailio-kemi-framework/docs/modules/kx/kx.get_duri.md

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+Return the value of destination URI ($du).
+

kamailio-kemi-framework/docs/modules/kx/kx.get_fhost.md

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+Return From-URI domain ($fd).
+

kamailio-kemi-framework/docs/modules/kx/kx.get_furi.md

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+Return the From URI($fu).
+

kamailio-kemi-framework/docs/modules/kx/kx.get_fuser.md

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+Return the From-URI username ($fU).
+

kamailio-kemi-framework/docs/modules/kx/kx.get_method.md

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+Return the SIP method ($rm).