Improved docs infer_datetime_format

Fixes pandas-dev#12152

Author: robintw

pandas/io/parsers.py

@@ -132,8 +132,10 @@ class ParserWarning(Warning):
 
     Note: A fast-path exists for iso8601-formatted dates.
 infer_datetime_format : boolean, default False
-    If True and parse_dates is enabled for a column, attempt to infer
-    the datetime format to speed up the processing
+    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.
 keep_date_col : boolean, default False
     If True and parse_dates specifies combining multiple columns then
     keep the original columns.