Add from_chars floating point docs

doc/charconv/from_chars.adoc

@@ -23,6 +23,9 @@ template <typename Integral>
 BOOST_CXX14_CONSTEXPR from_chars_result from_chars(const char* first, const char* last, Integral& value, int base = 10) noexcept;
 
 BOOST_CXX14_CONSTEXPR from_chars_result from_chars<bool>(const char* first, const char* last, bool& value, int base) = delete;
+
+template <typename Real>
+from_chars_result from_chars(const char* first, const char* last, Real& value, boost::charconv::chars_format fmt = boost::charconv::chars_format::general);
 ----
 
 == from_chars_result
@@ -37,18 +40,22 @@ BOOST_CXX14_CONSTEXPR from_chars_result from_chars<bool>(const char* first, cons
 == from_chars
 * first, last - valid range to parse
 * value - where the output is stored upon successful parsing
-* base - the integer base to use. Must be between 2 and 36 inclusive
+* base (integer only) - the integer base to use. Must be between 2 and 36 inclusive
+* fmt (floating point only) - The format of the buffer. See xref:chars_format.adoc[chars_format overview] for description.
+
+=== from_chars for integral types
+* All built-in integral types are allowed except bool which is deleted
+* These functions have been tested to support `__int128` and `unsigned __int128`
 * from_chars for integral types is constexpr when compiled using `-std=c++14` or newer
 ** One known exception is GCC 5 which does not support constexpr comparison of const char*.
 
-== from_chars for integral types
-* All built-in integral types are allowed except bool which is deleted
-* These functions have been tested to support `__int128` and `unsigned __int128` when compiling with `-std=gnu++11` or newer
+=== from_chars for floating point types
 
 == Examples
 
 === Basic usage
 [source, c++]
+==== Integral
 ----
 const char* buffer = "42";
 int v = 0;
@@ -56,9 +63,18 @@ from_chars_result r = boost::charconv::from_chars(buffer, buffer + std::strlen(b
 assert(r.ec == 0);
 assert(v == 42);
 ----
+==== Floating Point
+----
+const char* buffer = "1.2345"
+double v = 0;
+auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);
+assert(r.ec == 0);
+assert(v == 1.2345);
+----
 
 === Hexadecimal
 [source, c++]
+==== Integral
 ----
 const char* buffer = "2a";
 unsigned v = 0;
@@ -66,18 +82,31 @@ auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v, 16
 assert(r.ec == 0);
 assert(v == 42);
 ----
+==== Floating Point
+----
+const char* buffer = "1.3a2bp-10";
+double v = 0;
+auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v, boost::charconv::chars_format::hex);
+assert(r.ec == 0);
+assert(v == 8.0427e-18);
+----
+
 === EINVAL
 [source, c++]
 ----
 const char* buffer = "-123";
 unsigned v = 0;
 auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);
-assert(r.ec, EINVAL);
-assert(v == 0);
+assert(r.ec == EINVAL);
+----
+----
+const char* buffer = "-1.573e-3";
+double v = 0;
+auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v, boost::charconv::chars_format::fixed);
+assert(r.ec == EINVAL);
 ----
 In the event of EINVAL v is not set by `from_chars`
 
-
 === ERANGE
 [source, c++]
 ----

doc/charconv/to_chars.adoc

@@ -42,7 +42,8 @@ to_chars_result(char* first, char* last, Real value, boost::charconv::chars_form
 * first, last - pointers to the character buffer
 * value - the value to be paresed into the buffer
 * base (integer only) - the integer base to use. Must be between 2 and 36 inclusive
-* fmt (float only) - the floating point format
+* fmt (float only) - the floating point format to use.
+See xref:chars_format.adoc[chars_format overview] for description.
 * precision (float only) - the number of decimal places required
 
 === to_chars for integral types