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/usr/share/zsh/help/umask is in zsh-common 5.1.1-1ubuntu2.

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The actual contents of the file can be viewed below.

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umask [ -S ] [ mask ]
       The umask is set to mask.  mask can be either an octal number or
       a  symbolic value as described in chmod(1).  If mask is omitted,
       the current value is printed.  The -S option causes the mask  to
       be  printed as a symbolic value.  Otherwise, the mask is printed
       as an octal number.  Note that in the symbolic form the  permis-
       sions you specify are those which are to be allowed (not denied)
       to the users specified.