Here is a proposal to add printing of description of command line flags.
These descriptions are present in sources but they are not passed to the Flag structure.

There is an existing flag "-Xprintflags" that prints all flags and the default values.
This proposal extands that feature.

Here is a sample output of the proposed flag (formatting may go wrong below because of word wrapping)

java  -server  -XprintFlagsWithComments
[Global flags]
ccstr AbortVMOnException                   =  {notproduct}   Call fatal if this exception is thrown.  Example: java -XX:AbortVMOnExcepion=java.lang.NullPointerException Foo
ccstr AbortVMOnExceptionMessage            =  {notproduct}   Call fatal if the exception pointed by AbortVMOnException has this messag.
uintx AdaptivePermSizeWeight               = 20               {product}   Weight for perm gen exponential resizing, between 0 and 100
uintx AdaptiveSizeDecrementScaleFactor     = 4                {product}   Adaptive size scale down factor for shrinking
uintx AdaptiveSizeMajorGCDecayTimeScale    = 10               {product}   Time scale over which major costs decay
uintx AdaptiveSizePausePolicy              = 0                {product}   Policy for changing generation size for pause goals
uintx AdaptiveSizePolicyCollectionCostMargin  = 50               {product}   If collection costs are within margin, reduce both by ful delta
uintx AdaptiveSizePolicyGCTimeLimitThreshold  = 5                   Number of consecutive collections before gc time limit fires
uintx AdaptiveSizePolicyInitializingSteps  = 20               {product}   Number of steps where heuristics is used before data is used
uintx AdaptiveSizePolicyOutputInterval     = 0                {product}   Collecton interval for printing information; zero => never
uintx AdaptiveSizePolicyReadyThreshold     = 5                   Number of collections before the adaptive sizing is started
uintx AdaptiveSizePolicyWeight             = 10               {product}   Weight given to exponential resizing, between 0 and 100
uintx AdaptiveSizeThroughPutPolicy         = 0                {product}   Policy for changeing generation size for throughput goals
uintx AdaptiveTimeWeight                   = 25               {product}   Weight given to time in adaptive policy, between 0 and 100
 bool AdjustConcurrency                    = false            {product}   call thr_setconcurrency at thread create time to avoid LWP sarvation on MP systems (For Solaris Only)
 bool AggressiveOpts                       = false            {product}   Enable aggressive optimizations - see arguments.cpp
 intx AliasLevel                           = 3                {product}   0 for no aliasing, 1 for oop/field/static/array split, 2 forclass split, 3 for unique instances
 intx AlignEntryCode                       = 4                   aligns entry code to specified value (in bytes)
 intx AllocatePrefetchDistance             = -1               {product}   Distance to prefetch ahead of allocation pointer
.......