Updated doc on version numbering to match current tag

docs/internals/versions.md

 Yii version numbering
 =====================
 
+Releases
+--------
+
 A.B.C
 
 A = For Yii2 it's always 2.
 B = Major version. Non-BC changes with upgrade instructions.
 C = BC changes and additions.
 
-A.B.CrcD
+Release candidates
+------------------
+
+A.B.C-rc
+A.B.C-rc2
+
+This is when we want to do a release candidate. RC number increments till we're getting a stable release with no
+critical bugs and backwards incompatibility reports.
+
+Alphas and betas
+----------------
+
+A.B.C-alpha
+A.B.C-alpha2
+
+Alphas are unstable versions where significant bugs may and probably do exist. API isn't fixed yet and may be changed
+significantly. `alpha2` etc. may or may not be released based on overall stability of code and API.
+
+A.B.C-beta
+A.B.C-beta2
 
-This is when we want to release release candidate. D is the RC number. Starts with 1 and increments till we're getting a stable release with no critical bugs and BC incompatibility reports.
\ No newline at end of file
+Beta is more or less stable with less bugs and API instability than alphas. There still could be changes in API but
+there should be a significant reason for it.