Fix invalid/unusual documentation comment syntax

Collectively, these trigger warnings in both Doxygen and
clang (-Wdocumentation-unknown-command).  Weirdly, @p in doxygen doesn't do
anything but print in a fixed-width font anyway, so just use Markdown quotes to
replace it which has the same effect (and reads nicer in the source).  The use
of @param like @p was invalid, so do the same to those.

jack.h

@@ -119,7 +119,7 @@ char *jack_get_client_name (jack_client_t *client) JACK_OPTIONAL_WEAK_EXPORT;
 
 /**
  * @return pointer to a string representation of the UUID for
- * a client named @param name . If no such client exists, return NULL
+ * a client named `name`. If no such client exists, return NULL
  *
  * @param client the client making the request
  * @param name   the name of the client whose UUID is desired
@@ -807,7 +807,7 @@ int jack_port_set_alias (jack_port_t *port, const char *alias) JACK_OPTIONAL_WEA
 int jack_port_unset_alias (jack_port_t *port, const char *alias) JACK_OPTIONAL_WEAK_EXPORT;
 
 /*
- * Get any aliases known for @port.
+ * Get any aliases known for `port`.
  *
  * @return the number of aliases discovered for the port
  */

metadata.h

@@ -128,7 +128,7 @@ typedef struct {
  * Free a description.
  *
  * @param desc a jack_description_t whose associated memory will all be released
- * @param free_description_itself if non-zero, then @param desc will also be passed to free()
+ * @param free_description_itself if non-zero, then `desc` will also be passed to free()
  */
 void
 jack_free_description (jack_description_t* desc, int free_description_itself);
@@ -212,12 +212,12 @@ typedef void (*JackPropertyChangeCallback)(jack_uuid_t            subject,
                                            void*                  arg);
 
 /**
- * Arrange for @p client to call @p callback whenever a property is created,
+ * Arrange for `client` to call `callback` whenever a property is created,
  * changed or deleted.
  *
  * @param client the JACK client making the request
  * @param callback the function to be invoked when a property change occurs
- * @param arg the argument to be passed to @param callback when it is invoked
+ * @param arg the argument to be passed to `callback` when it is invoked
  *
  * @return 0 success, -1 otherwise.
  */