Improve docs on transfer() + minimum_balance()

[cmichi]

Closes #538 by clearing up the confusion that lead to it.

@athei I think it makes sense to expose a getter for the subsistence_threshold from seal. Otherwise it's not possible to take this variable into consideration in the smart contract when calculating how much to transfer (like in the issue).

/// Subsistence threshold is the extension of the minimum balance (aka existential deposit) by the
/// tombstone deposit, required for leaving a tombstone.

(from here)

We already have the minimum_balance, so it seems consistent to expose the subsistence threshold as well. Wdyt?

[codecov-io]

Codecov Report

Merging #540 into master will decrease coverage by 14.60%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #540       +/-   ##
===========================================
- Coverage   83.15%   68.55%   -14.61%     
===========================================
  Files         155      155               
  Lines        6811     6811               
===========================================
- Hits         5664     4669      -995     
- Misses       1147     2142      +995     

Impacted Files	Coverage Δ
crates/env/src/api.rs	37.50% <ø> (ø)
crates/env/src/backend.rs	0.00% <ø> (ø)
crates/env/src/engine/off_chain/db/chain_spec.rs	100.00% <ø> (ø)
crates/lang/src/env_access.rs	0.00% <ø> (ø)
crates/lang/codegen/src/traits.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/ir/src/ir/contract.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/codegen/src/generator/env.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/codegen/src/generator/storage.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/codegen/src/generator/contract.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/codegen/src/generator/trait_def.rs	0.00% <0.00%> (-100.00%)	⬇️
... and 18 more

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[Robbepop]

LGTM

Thanks a lot for the docs improvements!