Add descriptions to soong_build actions.

[  0% 0/5 ] generating Soong docs at out/soong/docs/soong_build.html
0:03 analyzing Android.bp files and generating ninja file at out/soong/build.ninja
0:03 converting Android.bp files to BUILD files at out/soong/bp2build
0:03 generating the Soong module graph at out/soong/module-graph.json
0:03 generating the Soong module graph as a Bazel workspace at out/soong/queryview
0:03 generating Soong docs at out/soong/docs/soong_build.html

Test: m soong_docs json-module-graph bp2build queryview nothing
Change-Id: I33abb8828ead9ef999ffa1a6c9c4b016957c6dd1