time_shift documentation in trigger function page is not consistent. some examples :

• in some functions we refer function avg(), to keep the description in one place only, for example :
  "See function avg() for an example of using time_shift."
  in some we include an example and refer to avg() - like in last()
  in some we do not reference or explain anything - like in band()
  in some the reference is in a different format - like in delta()

• in most functions we mention the starting version like this :
  "time_shift parameter and string operators are supported from Zabbix 1.8.2."
  delta(), max(), min() and possibly others use a different sentence :
  "Since Zabbix 1.8.2, the function supports a second, optional parameter time_shift."

• we reference functions with empty parenthesis appended to their name - like avg()... but not everywhere. for example, avg() references in max(), min(), percentile() and possibly other functions have 'avg' only