to_timestamp docs

I don't think the changes made in PG 12 are documented accurately.  It
currently says:

    to_timestamp and to_date matches any single separator in the input
    string or is skipped

However, I think it is more accurate to say _multiple_ whitespace can
also be matched by a single separator:

    SELECT to_timestamp('%1976','_YYYY');
          to_timestamp
    ------------------------
     1976-01-01 00:00:00-05
    
    SELECT to_timestamp('%%1976','_YYYY');
    ERROR:  invalid value "%197" for "YYYY"
    DETAIL:  Value must be an integer.
    
    -- two spaces
-->    SELECT to_timestamp('  1976','_YYYY');
          to_timestamp
    ------------------------
     1976-01-01 00:00:00-05

-->    SELECT to_timestamp(E'\t\t\t1976','_YYYY');
          to_timestamp
    ------------------------
     1976-01-01 00:00:00-05

Proposed patch attached.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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