Updated docs

pandas/io/parsers.py

@@ -132,10 +132,10 @@ class ParserWarning(Warning):
 
     Note: A fast-path exists for iso8601-formatted dates.
 infer_datetime_format : boolean, default False
-    If parse_dates is enabled and this flag is set, pandas will attempt to
-    infer the format of the datetime strings in the columns, and if it can be
-    inferred, switch to a faster method of parsing them. In some cases this
-    can increase the parsing speed by ~5-10x.
+    If True and parse_dates is enabled, pandas will attempt to infer the format
+    of the datetime strings in the columns, and if it can be inferred, switch
+    to a faster method of parsing them. In some cases this can increase the
+    parsing speed by ~5-10x.
 keep_date_col : boolean, default False
     If True and parse_dates specifies combining multiple columns then
     keep the original columns.

pandas/tseries/tools.py

@@ -231,8 +231,10 @@ def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
     unit : unit of the arg (D,s,ms,us,ns) denote the unit in epoch
         (e.g. a unix timestamp), which is an integer/float number.
     infer_datetime_format : boolean, default False
-        If no `format` is given, try to infer the format based on the first
-        datetime string. Provides a large speed-up in many cases.
+        If True and no `format` is given, attempt to infer the format of the
+        datetime strings, and if it can be inferred, switch to a faster
+        method of parsing them. In some cases this can increase the parsing
+        speed by ~5-10x.
 
     Returns
     -------