doc: Correct documentation of install-time environment variables

Since approximately PostgreSQL 10, it is no longer required that
environment variables at installation time such as PERL, PYTHON, TCLSH
be "full path names", so change that phrasing in the installation
instructions.  (The exact time of change appears to differ for PERL
and the others, but it works consistently in PostgreSQL 10.)

Also while we're here document the defaults for PERL and PYTHON, but
since the search list for TCLSH is so long, let's leave that out so we
don't need to maintain a copy of that list in the installation
instructions.

Author: petere

doc/src/sgml/installation.sgml

@@ -1500,8 +1500,9 @@ su - postgres
        <term><envar>PERL</envar></term>
        <listitem>
         <para>
-         Full path name of the Perl interpreter.  This will be used to
-         determine the dependencies for building PL/Perl.
+         Perl interpreter program.  This will be used to determine the
+         dependencies for building PL/Perl.  The default is
+         <command>perl</command>.
         </para>
        </listitem>
       </varlistentry>
@@ -1510,13 +1511,13 @@ su - postgres
        <term><envar>PYTHON</envar></term>
        <listitem>
         <para>
-         Full path name of the Python interpreter.  This will be used to
+         Python interpreter program.  This will be used to
          determine the dependencies for building PL/Python.  Also,
          whether Python 2 or 3 is specified here (or otherwise
          implicitly chosen) determines which variant of the PL/Python
          language becomes available.  See
          <xref linkend="plpython-python23"/>
-         for more information.
+         for more information.  The default is <command>python</command>.
         </para>
        </listitem>
       </varlistentry>
@@ -1525,7 +1526,7 @@ su - postgres
        <term><envar>TCLSH</envar></term>
        <listitem>
         <para>
-         Full path name of the Tcl interpreter.  This will be used to
+         Tcl interpreter program.  This will be used to
          determine the dependencies for building PL/Tcl, and it will
          be substituted into Tcl scripts.
         </para>