Improve QDoubleValidator's docs

Mention that the C locale is an exception on group separators.
Rewrite a "It is thus recommended to use" - which is garblage induced
by addiction to passive voice ("We recommend use of ..." would be
better) - to actually say what needs to be said.

Actually say what the standard and scientific notations are, explain
which parts are locale-dependent, \sa some relevant QLocale docs (from
which the other relevant docs are easily reached).

Rephrase the explanations for validate()'s returns. Address validity
of the string before constraints on the value it wouldn't represent if
it weren't valid.

Change-Id: I5e677f00e5146a44eea5b08c5e0146cd235b95b1
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/gui/util/qvalidator.cpp

@@ -531,10 +531,11 @@ public:
     in the German locale, "1,234" will be accepted as the fractional number
     1.234. In Arabic locales, QDoubleValidator will accept Arabic digits.
 
-    \note The QLocale::NumberOptions set on the locale() also affect the
-    way the number is interpreted. For example, since QLocale::RejectGroupSeparator
-    is not set by default, the validator will accept group separators. It is thus
-    recommended to use QLocale::toDouble() to obtain the numeric value.
+    \note The QLocale::NumberOptions set on the locale() also affect the way the
+    number is interpreted. For example, since QLocale::RejectGroupSeparator is
+    not set by default (except on the \c "C" locale), the validator will accept
+    group separators. If the string passes validation, pass it to
+    locale().toDouble() to obtain its numeric value.
 
     \sa QIntValidator, QRegularExpressionValidator, QLocale::toDouble(), {Line Edits Example}
 */
@@ -544,10 +545,23 @@ public:
     \since 4.3
     This enum defines the allowed notations for entering a double.
 
-    \value StandardNotation      The string is written as a standard number
-                                 (i.e. 0.015).
-    \value ScientificNotation    The string is written in scientific
-                                 form. It may have an exponent part(i.e. 1.5E-2).
+    \value StandardNotation The string is written in the standard format, a
+                            whole number part optionally followed by a separator
+                            and fractional part, for example \c{"0.015"}.
+
+    \value ScientificNotation The string is written in scientific form, which
+                              optionally appends an exponent part to the
+                              standard format, for example \c{"1.5E-2"}.
+
+    The whole number part may, as usual, include a sign. This, along with the
+    separators for fractional part, exponent and any digit-grouping, depend on
+    locale. QDoubleValidator doesn't check the placement (which would also
+    depend on locale) of any digit-grouping separators it finds, but it will
+    reject input that contains them if \l QLocale::RejectGroupSeparator is set
+    in \c locale().numberOptions().
+
+    \sa QLocale::numberOptions(), QLocale::decimalPoint(),
+        QLocale::exponential(), QLocale::negativeSign()
 */
 
 /*!
@@ -589,14 +603,14 @@ QDoubleValidator::~QDoubleValidator()
 /*!
     \fn QValidator::State QDoubleValidator::validate(QString &input, int &pos) const
 
-    Returns \l Acceptable if the string \a input contains a double
-    that is within the valid range and is in the correct format.
+    Returns \l Acceptable if the string \a input is in the correct format and
+    contains a double within the valid range.
 
-    Returns \l Intermediate if \a input contains a double that is
-    outside the range or is in the wrong format; e.g. is empty.
+    Returns \l Intermediate if \a input is in the wrong format or contains a
+    double outside the range.
 
-    Returns \l Invalid if the \a input is not a double or with too many
-    digits after the decimal point.
+    Returns \l Invalid if the \a input doesn't represent a double or has too
+    many digits after the decimal point.
 
     Note: If the valid range consists of just positive doubles (e.g. 0.0 to 100.0)
     and \a input is a negative double then \l Invalid is returned. If notation()