conf: add option to set maximum slew rate

With the generic driver, the maxslewrate directive sets the maximum
frequency offset that the driver is allowed to use to slew the time. By
default, it's set to 83333.333 (1/12). This is identical to what Linux
fast slewing used to use.

chrony.texi.in

@@ -1155,6 +1155,7 @@ the configuration file is ignored.
 * maxchange directive::         Set maximum allowed offset
 * maxclockerror directive::     Set maximum frequency error of local clock
 * maxsamples directive::        Set maximum number of samples per source
+* maxslewrate directive::       Set maximum slew rate
 * maxupdateskew directive::     Stop bad estimates upsetting machine clock
 * minsamples directive::        Set minimum number of samples per source
 * noclientlog directive::       Prevent chronyd from gathering data about clients
@@ -1536,8 +1537,10 @@ The syntax is
 corrtimeratio 100
 @end example
 
-The current remaining correction is shown in the @code{tracking} report
-(@pxref{tracking command}) as the @code{System time} value.
+The maximum allowed slew rate can be set by the @code{maxslewrate}
+directive (@pxref{maxslewrate directive}.  The current remaining
+correction is shown in the @code{tracking} report (@pxref{tracking
+command}) as the @code{System time} value.
 @c }}}
 @c {{{ deny
 @node deny directive
@@ -2398,6 +2401,23 @@ The syntax is
 maxsamples <samples>
 @end example
 @c }}}
+@c {{{ maxslewrate
+@node maxslewrate directive
+@subsection maxslewrate
+The @code{maxslewrate} directive sets the maximum rate at which @code{chronyd}
+is allowed to slew the time.  It limits the slew rate controlled by the
+correction time ratio (@pxref{corrtimeratio directive}) and is effective
+only on systems where @code{chronyd} is able to control the rate (so
+far only Linux).
+
+By default, the maximum slew rate is 83333.333 ppm (one twelfth).
+
+The syntax is
+
+@example
+maxslewrate <rate-in-ppm>
+@end example
+@c }}}
 @c {{{ maxupdateskew
 @node maxupdateskew directive
 @subsection maxupdateskew