documentation added for web3-eth-admin module & pendingTransactions

[princesinha19]

Description

I have added the documentation for web3-eth-admin module and eth.pendingTranactions.
It's dependent to #2655, #2657.

• Documentation

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no warnings.
• I have updated or added types for all modules I've changed
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test in the root folder with success and extended the tests if necessary.
• I ran npm run build in the root folder and tested it in the browser and with node.
• I ran npm run dtslint in the root folder and tested that all my types are correct
• I have tested my code on an ethereum test network.

[coveralls]

Coverage remained the same at 95.374% when pulling e3e425d on princesinha19:document_web3 into 839283a on ethereum:1.0.

[coveralls]

Coverage remained the same at 95.374% when pulling e3e425d on princesinha19:document_web3 into 839283a on ethereum:1.0.

[coveralls]

Coverage remained the same at 95.376% when pulling 50f96b4 on princesinha19:document_web3 into be9582b on ethereum:1.0.

[nivida]

I will extend the docs a bit with some additional details. @princesinha19

[princesinha19]

okay, cool @nivida.

[princesinha19]

Done all the changes.