VERSIONS: clarify our versioning concept

Drop the pre-release part from this text as we don't use that in
practise since many years.

Update the phrasing to reflect our more strict interpretation:
http://curl.haxx.se/mail/lib-2011-08/0064.html

docs/VERSIONS

@@ -11,32 +11,25 @@ Version Numbers and Releases
 
  The version numbering is always built up using the same system:
 
-        X.Y[.Z][-preN]
+        X.Y[.Z]
 
  Where
    X is main version number
    Y is release number
    Z is patch number
-   N is pre-release number
 
  One of these numbers will get bumped in each new release. The numbers to the
  right of a bumped number will be reset to zero. If Z is zero, it may not be
- included in the version number. The pre release number is only included in
- pre releases (they're never used in public, official, releases).
+ included in the version number.
 
  The main version number will get bumped when *really* big, world colliding
- changes are made. The release number is bumped when big changes are
- performed. The patch number is bumped when the changes are mere bugfixes and
- only minor feature changes. The pre-release is a counter, to identify which
- pre-release a certain release is.
-
- When reaching the end of a pre-release period, the version without the
- pre-release part will be released as a public release.
+ changes are made. The release number is bumped when changes are performed or
+ things/features are added. The patch number is bumped when the changes are
+ mere bugfixes.
 
  It means that after release 1.2.3, we can release 2.0 if something really big
  has been made, 1.3 if not that big changes were made or 1.2.4 if mostly bugs
- were fixed. Before 1.2.4 is released, we might release a 1.2.4-pre1 release
- for the brave people to try before the actual release.
+ were fixed.
 
  Bumping, as in increasing the number with 1, is unconditionally only
  affecting one of the numbers (except the ones to the right of it, that may be
@@ -56,12 +49,12 @@ Version Numbers and Releases
         #define LIBCURL_VERSION_NUM 0xXXYYZZ
 
  Where XX, YY and ZZ are the main version, release and patch numbers in
- hexadecimal. All three numbers are always represented using two digits.  1.2
- would appear as "0x010200" while version 9.11.7 appears as "0x090b07".
+ hexadecimal. All three number fields are always represented using two digits
+ (eight bits each). 1.2 would appear as "0x010200" while version 9.11.7
+ appears as "0x090b07".
 
- This 6-digit hexadecimal number does not show pre-release number, and it is
- always a greater number in a more recent release. It makes comparisons with
- greater than and less than work.
+ This 6-digit hexadecimal number is always a greater number in a more recent
+ release. It makes comparisons with greater than and less than work.
 
  This number is also available as three separate defines:
  LIBCURL_VERSION_MAJOR, LIBCURL_VERSION_MINOR and LIBCURL_VERSION_PATCH.