fix: add rpc help text for "ehf" field (#5628)

## Issue being fixed or feature implemented
#5597 follow-up 

## What was done?
add missing filed description

## How Has This Been Tested?
`help getblockchaininfo`

## Breaking Changes
n/a

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

src/rpc/blockchain.cpp

@@ -1672,6 +1672,7 @@ UniValue getblockchaininfo(const JSONRPCRequest& request)
                         {RPCResult::Type::NUM, "bit", "the bit (0-28) in the block version field used to signal this softfork (only for \"started\" status)"},
                         {RPCResult::Type::NUM_TIME, "start_time", "the minimum median time past of a block at which the bit gains its meaning"},
                         {RPCResult::Type::NUM_TIME, "timeout", "the median time past of a block at which the deployment is considered failed if not yet locked in"},
+                        {RPCResult::Type::NUM, "ehf", "the minimum height when miner's signals for the deployment can be accepted (special values: \"-1\" - any, \"0\" - none)"},
                         {RPCResult::Type::NUM, "since", "height of the first block to which the status applies"},
                         {RPCResult::Type::OBJ, "statistics", "numeric statistics about BIP9 signalling for a softfork",
                         {