fastboot: add documentation

add exclude-dynamic partitions command to show_help

Test: th
Change-Id: I3941080382c258fa418f413a25fdbaef4b70ce76
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 156dc3b..1c52da2 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -638,7 +638,10 @@
             " --disable-verification     Sets disable-verification when flashing vbmeta.\n"
             " --disable-super-optimization\n"
             "                            Disables optimizations on flashing super partition.\n"
-            " --disable-fastboot-info    Will collects tasks from image list rather than $OUT/fastboot-info.txt.\n"
+            " --exclude-dynamic-partitions\n"
+            "                            Excludes flashing of dynamic partitions.\n"
+            " --disable-fastboot-info    Will collects tasks from image list rather than \n"
+            "                            $OUT/fastboot-info.txt.\n"
             " --fs-options=OPTION[,OPTION]\n"
             "                            Enable filesystem features. OPTION supports casefold, projid, compress\n"
             // TODO: remove --unbuffered?