Some of these are grammar/spelling comments, and some of these are more
stylistic.

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 > The expression at its right is expressed using almost standard formulae.

I'm not sure what this refers to. Is this the inside of (expr ...)?

 > A previous SRFI, SRFI 105, Curly-infix-expressions, already introduced …

I would omit "already" since "previous" is used here.

 > The current SRFI …

I would say "this current SRFI."

 > According to SRFI 261…

I would make more explicit that "expr" is the SRFI 97/SRFI 261 name for
this library. It should also be added to the metadata so it shows up in
the SRFI list.

 > syntaxic

Every instance of this should be replaced with "syntactic." This word
means something different.[1]

 > Operators are of one of the below types:

Should omit "of" after "are of".

 > The lower the priority number is, the most prioritized the operator.

"most" should be "more."

 > the predefined operators + and * have respective priority numbers of
50 and 40

I would say "the predefined operators + and * have priority numbers of
50 and 40 respectively."

 > Calls are made of two parts: the called and the arguments

I would prefer "callee" because that is a noun, while "the called" is
harder to parse. I would also mark-up "called" and "arguments" (or
"callee" and "arguments") in italics to make it easier to read.

 > evaluating to a callable lambda

Technically, not all callable objects are lambdas, and all lambdas are
callable. I would write "evaluating to a procedure."

 > depending one the context

Should be "depending on."

 > Types of operators that can be defined are:

Should be "The types of operators that can be defined are…" (no colon)

 > OPERATOR X becomes (OPERATION X).

I think you should add something like "each operator is associated with
an operation, which is a procedure [add more explanation here]."

 > When argument of a call, …

This should have a subject, like "When the subexpression is the argument
of a call, …"

 >  it expands to the inner subexpression, literally.

I would remove the comma before "literally."

 >  Then ((a ...)) becomes …

"Then" is a bit awkward here, because "((a ...))" was not introduced
beforehand. I would write "in other words."

 > in the below table:

I would write "table below:"

 > using internals of implementations

I would write "using the internals of implementations."

[1]: https://www.merriam-webster.com/dictionary/syntaxic