[DOCS][PYTHON] Update docs for clarity (#531)

I only modified those docs pages that are untouched in existing unmerged
PRs, so hopefully there are no merge conflicts!

1. The `tantivy-py` version specified in the docs doesn't work (pip
install fails), but with the latest version of pip and wheel installed
on my Mac, I was able to just `pip install tantivy` and FTS works great
for me. I updated the docs page to include this in
7ca4b757ce but can always modify to
another specific version in case this breaks any tests.
2. The `.add()` method for Python should take in a list of dicts as the
first option (to also align with the JS API), and additionally, users
can pass an existing pandas DataFrame to add to a table. Hope this makes
sense.
3. I've had multiple conversations with users who are unclear that the
terms "exhaustive", "flat" and "KNN" are all the same kind of search, so
I've updated the verbiage of this section to clarify this.
4. Fixed typos and improved clarity in the ANN indexes page.

docs/src/fts.md

@@ -6,17 +6,19 @@ to make this available for JS as well.
 
 ## Installation
 
-To use full text search, you must install optional dependency tantivy-py:
+To use full text search, you must install the dependency `tantivy-py`:
 
-# tantivy 0.19.2
-pip install tantivy@git+https://github.com/quickwit-oss/tantivy-py#164adc87e1a033117001cf70e38c82a53014d985
+# tantivy 0.20.1
+```sh
+pip install tantivy==0.20.1
+```
 
 
 ## Quickstart
 
 Assume:
 1. `table` is a LanceDB Table
-2. `text` is the name of the Table column that we want to index
+2. `text` is the name of the `Table` column that we want to index
 
 For example,