Improve manpage regarding the new custom acceleration profile

Commits f94a8edb and 484b6a7f added a new custom acceleration profile. They also included some entries to the libinput.4 manpage. However, there are still a few issues in the manpage:

• The new Xorg configuration entries (Option "…") were documented, but the corresponding libinput runtime properties are still not documented:
  • libinput Accel Profile Enabled now has 3 values instead of 2.
  • libinput Accel Profile Enabled Default now has 3 values instead of 2.
  • libinput Accel Profiles Available now has 3 values instead of 2.
  • libinput Accel Custom Fallback Points is undocumented
  • libinput Accel Custom Fallback Step is undocumented
  • libinput Accel Custom Motion Points is undocumented
  • libinput Accel Custom Motion Step is undocumented
  • libinput Accel Custom Scroll Points is undocumented
  • libinput Accel Custom Scroll Step is undocumented
• The current manpage literally says “see the libinput documentation” several times. Well, the manpage IS documentation, and telling the reader to read the documentation that the user is already reading is not helpful. So, either fully document it at the manpage, or point to the other location where the rest of the documentation can be found.
• Since there are now more options related to the custom acceleration than other acceleration options, it is easier to group them together and describe them all at once, like this:

[…]
.TP 7
.BI "Option \*qAccelSpeed\*q \*q" float \*q
Sets the pointer acceleration speed within the range [-1, 1].
This only applies to the flat or adaptive profile.
.TP 7
.BI "Option \*qAccelPointsFallback\*q \*q" string \*q
.TQ
.BI "Option \*qAccelPointsMotion\*q \*q" string \*q
.TQ
.BI "Option \*qAccelPointsScroll\*q \*q" string \*q
Sets the points of the Fallback/Motion/Scroll acceleration functions.
The string must be a space-separated list of floating point non-negative numbers, e.g.
"0.0 1.0 2.4 2.5".
This only applies to the custom profile.
.TP 7
.BI "Option \*qAccelStepFallback\*q \*q" float \*q
.TQ
.BI "Option \*qAccelStepMotion\*q \*q" float \*q
.TQ
.BI "Option \*qAccelStepScroll\*q \*q" float \*q
Sets the step between the points of the Fallback/Motion/Scroll acceleration functions.
When a step of 0.0 is provided, libinput's default Fallback acceleration function is used.
This only applies to the custom profile.
.TP 7
.BI "Option \*qButtonMapping\*q \*q" string \*q
[…]

which renders like:

Since @whot and @Yinon worked on the feature, they are the people with most knowledge about the new options, and I think they should improve/fix the manpage.