doc: Replace remaining literal BTC with CURRENCY_UNIT.

[domob1812]

This replaces one remaining instance of the literal "BTC" string with the CURRENCY_UNIT constant, as is done in most of the codebase already.

After this change, no instance of literal "BTC" remains anywhere in the RPC help texts.

[maflcko]

I think this is ready to merge if it compiles

[practicalswift]

Concept ACK

FWIW:

$ git grep BTC -- "*.cpp" "*.h" ":(exclude)src/qt/" ":(exclude)src/wallet/test/" ":(exclude)src/test/" | grep -vE ': *(//|\*)'
src/policy/feerate.cpp:const std::string CURRENCY_UNIT = "BTC";
src/validation.cpp:        LogPrint(BCLog::MEMPOOL, "replacing tx %s with %s for %s BTC additional fees, %d delta bytes\n",
src/wallet/rpcwallet.cpp:            "                         Must be at least 0.0001 BTC per kB higher than the current transaction fee rate.\n"},

[domob1812]

Concept ACK

FWIW:

$ git grep BTC -- "*.cpp" "*.h" ":(exclude)src/qt/" ":(exclude)src/wallet/test/" ":(exclude)src/test/" | grep -vE ': *(//|\*) .*'
src/policy/feerate.cpp:const std::string CURRENCY_UNIT = "BTC";
src/validation.cpp:        LogPrint(BCLog::MEMPOOL, "replacing tx %s with %s for %s BTC additional fees, %d delta bytes\n",
src/wallet/rpcwallet.cpp:            "                         Must be at least 0.0001 BTC per kB higher than the current transaction fee rate.\n"},

Good catch (I was only targeting the RPC help texts so far, as that is what I worked on when I noticed this). I've updated the PR, replacing also the validation.cpp instance (and squashed the commits).

[hebasto]

Concept ACK.

[maflcko]

ACK 7df0cf7

[laanwj]

ACK 7df0cf7
Doesn't change any translation strings so can go into 0.20.0.