pgsql: doc: Correct documentation of install-time environment variables

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.

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c559d8dbf9e7871c64ecc976199cc89765a6cee8

Modified Files
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doc/src/sgml/installation.sgml | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)